UPDATE
Finally resolved the issue with Shelly 2 Dimmer blowing D1 & D14 (IN4007) twice!
With fantastic help from thgoebel and doing some further testing. The Shelly 2 Dimmer does appear prone to high voltage inductive spiking. Nothing showed while using a digital volt meter when devices on the same power line were switched on or off. Closer inspection using a mains analyser I could see HF spiking of >300v on the line when devices were switched on (or off) I assume this was the root cause these diodes fail on the Shelly 2 Dimmers (poor diode response time to HF spiking).
I have since fitted a small two stage mains filter prior to the dimmer which has totally eliminated all signs of spiking on the mains line to the unit, replaced the Shelly 2 Dimmer with the Shelly Dimmer Gen3 which also has additional mains filtering built in. Hopefully this is the end of fry ups! watch this space ![]()