Controlling hot water immersion using iboost and Shelly 1PB Gen 4

[jrwm47]

I have been using predbat with a 13k5Wh GE AIO and 3k5W solar for around 9 months. I am so pleased with it that, now my oil boiler is switched off for the summer, I would like to use iboost to control a Shelly1PM Gen4 for the immersion heater, using PV or Agile, as available.

I have tried using the predbat Docs sample iboost YAML with Shelly integration [switch.shelly1pmg4_a085e3bc7220_switch_0] but HA always objects "500". As anyone suceeded with this arrangement? If so, can you share your Automation?
Julian