Speaking from a 'Sparks' perspective Leon. I would suggest you get a 'Spark' in to check, just to make sure.
It could be anything. Damn hard, (sight unseen).
Might be the frequency? of your dew heater but IT should not give you a tingle. (Voltage is too low). I take it, that IT IS running off 12 Volts???
One thing you may want to install, if not already, is an 'Earth Leakage Circuit Breaker'. These devices will protect against all possible faults. They can be bought as a 'Plug in' as well. Place it before your heavy extension lead.
Can't help much more Leon.
Get an electrician in..
Regards, L.
