diff --git a/DigitalLearningSolutions.Data/DataServices/CourseDataService.cs b/DigitalLearningSolutions.Data/DataServices/CourseDataService.cs
index 9f0ab58aec..df086021b6 100644
--- a/DigitalLearningSolutions.Data/DataServices/CourseDataService.cs
+++ b/DigitalLearningSolutions.Data/DataServices/CourseDataService.cs
@@ -139,7 +139,7 @@ int EnrolOnActivitySelfAssessment(int selfAssessmentId, int candidateId, int sup
         public IEnumerable<DelegateAssessmentStatistics> GetDelegateAssessmentStatisticsAtCentre(string searchString, int centreId, string categoryName, string isActive, int? categoryId);
 
         bool IsSelfEnrollmentAllowed(int customisationId);
-        Customisation? GetCourse(int customisationId);
+        Customisation? GetCourse(int? customisationId);
     }
 
     public class CourseDataService : ICourseDataService
@@ -2024,7 +2024,7 @@ public bool IsSelfEnrollmentAllowed(int customisationId)
             return selfRegister > 0;
         }
 
-        public Customisation? GetCourse(int customisationId)
+        public Customisation? GetCourse(int? customisationId)
         {
             return connection.Query<Customisation>(
                 @"SELECT CustomisationID
diff --git a/DigitalLearningSolutions.Data/Helpers/FilteringHelper.cs b/DigitalLearningSolutions.Data/Helpers/FilteringHelper.cs
index 4ea7ae9ebe..a806df3f5d 100644
--- a/DigitalLearningSolutions.Data/Helpers/FilteringHelper.cs
+++ b/DigitalLearningSolutions.Data/Helpers/FilteringHelper.cs
@@ -66,8 +66,7 @@ public static string BuildFilterValueString(string group, string propertyName, s
                 ? defaultFilterValue
                 : AddNewFilterToFilterString(existingFilterString, newFilterToAdd);
         }
-
-        public static string? GetCategoryAndTopicFilterString(
+       public static string? GetCategoryAndTopicFilterString(
             string? categoryFilterString,
             string? topicFilterString
         )
@@ -168,7 +167,59 @@ public static string GetFilterValueForRegistrationPrompt(int promptNumber, strin
                 : answer;
             return BuildFilterValueString(group, group.Split('(')[0], propertyValue);
         }
+        public  static  string? GetValidFilters(string existingFilterString, string newFilterToAdd, IEnumerable<FilterModel> availableFilters, HttpRequest request, string cookieName)
+        {
+            var cookieValue = request.Cookies[cookieName];
+            if (string.IsNullOrEmpty(cookieValue) || cookieValue == EmptyFiltersCookieValue)
+            {
+                return existingFilterString;
+            }
+            var existingFilters = cookieValue.Split(FilterSeparator);
+            var validFilterValues = availableFilters
+                .SelectMany(filter => filter.FilterOptions)
+                .Select(option => option.FilterValue)
+                .ToHashSet();
+           
+            var filteredResults = existingFilters
+        .Where(entry => IsFilterInvalid(entry, validFilterValues))
+        .ToList();
+            var newCookieValue = string.Join(FilterSeparator, filteredResults);
+            if (string.IsNullOrEmpty(newCookieValue)) return null;
+            newCookieValue = AddNewFilterToFilterString(newCookieValue, newFilterToAdd);
+            return RemoveDuplicateFilters( newFilterToAdd, newCookieValue);
+        }
 
+        private static  bool IsFilterInvalid(string filterEntry, HashSet<string> validFilterValues)
+        {
+            if (validFilterValues.Contains(filterEntry)) return true;
+            return false;
+        }
+        public  static string RemoveDuplicateFilters(string newFilterToAdd, string? existingFilterString)
+        {
+            if (string.IsNullOrEmpty(existingFilterString))
+            {
+                return existingFilterString ?? string.Empty;
+            }
+            var selectedFilters = existingFilterString.Split(FilteringHelper.FilterSeparator).ToList();
+            if (!string.IsNullOrEmpty(newFilterToAdd))
+            {
+                var filterHeader = newFilterToAdd.Split(FilteringHelper.Separator)[0];
+                var dupfilters = selectedFilters.Where(x => x.Contains(filterHeader));
+                if (dupfilters.Count() > 1)
+                {
+                    foreach (var filter in selectedFilters)
+                    {
+                        if (filter.Contains(filterHeader))
+                        {
+                            selectedFilters.Remove(filter);
+                            existingFilterString = string.Join(FilteringHelper.FilterSeparator, selectedFilters);
+                            break;
+                        }
+                    }
+                }
+            }
+            return existingFilterString;
+        }
         private static IEnumerable<FilterOptionModel> GetFilterOptionsForPromptWithOptions(Prompt prompt)
         {
             var group = GetFilterGroupForPrompt(prompt);
diff --git a/DigitalLearningSolutions.Web/Controllers/TrackingSystem/Delegates/ActivityDelegatesController.cs b/DigitalLearningSolutions.Web/Controllers/TrackingSystem/Delegates/ActivityDelegatesController.cs
index bfa04cd271..eea76c00d7 100644
--- a/DigitalLearningSolutions.Web/Controllers/TrackingSystem/Delegates/ActivityDelegatesController.cs
+++ b/DigitalLearningSolutions.Web/Controllers/TrackingSystem/Delegates/ActivityDelegatesController.cs
@@ -95,8 +95,7 @@ public IActionResult Index(
 
             sortBy ??= DefaultSortByOptions.Name.PropertyName;
             sortDirection ??= GenericSortingHelper.Ascending;
-
-            existingFilterString = FilteringHelper.GetFilterString(
+                existingFilterString = FilteringHelper.GetFilterString(
                 existingFilterString,
                 newFilterToAdd,
                 clearFilters,
@@ -142,25 +141,7 @@ public IActionResult Index(
             if (!string.IsNullOrEmpty(existingFilterString))
             {
                 var selectedFilters = existingFilterString.Split(FilteringHelper.FilterSeparator).ToList();
-
-                if (!string.IsNullOrEmpty(newFilterToAdd))
-                {
-                    var filterHeader = newFilterToAdd.Split(FilteringHelper.Separator)[0];
-                    var dupfilters = selectedFilters.Where(x => x.Contains(filterHeader));
-                    if (dupfilters.Count() > 1)
-                    {
-                        foreach (var filter in selectedFilters)
-                        {
-                            if (filter.Contains(filterHeader))
-                            {
-                                selectedFilters.Remove(filter);
-                                existingFilterString = string.Join(FilteringHelper.FilterSeparator, selectedFilters);
-                                break;
-                            }
-                        }
-                    }
-                }
-
+                existingFilterString = FilteringHelper.RemoveDuplicateFilters(newFilterToAdd, existingFilterString);
                 if (selectedFilters.Count > 0)
                 {
                     foreach (var filter in selectedFilters)
@@ -276,7 +257,10 @@ public IActionResult Index(
                 var activityName = isCourseDelegate
                     ? courseService.GetCourseNameAndApplication((int)customisationId).CourseName
                     : selfAssessmentService.GetSelfAssessmentNameById((int)selfAssessmentId);
-
+                if (!string.IsNullOrEmpty(existingFilterString))
+                {
+                    existingFilterString = FilteringHelper.GetValidFilters( existingFilterString, newFilterToAdd, availableFilters, Request, filterCookieName);
+                }
                 if (isCourseDelegate)
                 {
                     var result = paginateService.Paginate(
@@ -591,6 +575,7 @@ DelegateAccessRoute accessedVia
                 return RedirectToAction("Index", "ActivityDelegates", routeData, returnPageQuery.Value.ItemIdToReturnTo);
             }
         }
+
     }
 }
 
diff --git a/DigitalLearningSolutions.Web/Services/CourseService.cs b/DigitalLearningSolutions.Web/Services/CourseService.cs
index 9c255baf2e..530d34c22c 100644
--- a/DigitalLearningSolutions.Web/Services/CourseService.cs
+++ b/DigitalLearningSolutions.Web/Services/CourseService.cs
@@ -134,7 +134,7 @@ int diagCompletionThreshold
         int GetNumberOfActiveCoursesAtCentreFilteredByCategory(int centreId, int? categoryId);
         public IEnumerable<Course> GetApplicationsAvailableToCentre(int centreId);
         bool IsSelfEnrollmentAllowed(int customisationId);
-        Customisation? GetCourse(int customisationId);
+        Customisation? GetCourse(int? customisationId);
     }
 
     public class CourseService : ICourseService
@@ -632,7 +632,7 @@ public bool IsSelfEnrollmentAllowed(int customisationId)
             return courseDataService.IsSelfEnrollmentAllowed(customisationId);
         }
 
-        public Customisation? GetCourse(int customisationId)
+        public Customisation? GetCourse(int? customisationId)
         {
             return courseDataService.GetCourse(customisationId);
         }