Cloudy day today so not hot water again so had to unplug the controller. So while it was out I bit the bullet and pulled it apart and added an LED and momentary Push Button switch.

D1 - PWM output to Triac Driver

D2 - Indicator LED

D3 - PB Switch

Due to my bad code with lots of delays in the main loop - eek I'm going to have a go at using the button to trigger and interrupt to override the controller and give me full power for 1 hour (that's about how long it takes to heat the tank from cold).

After the hour the system will revert back to normal operation.