If you dont feel confident, then I would say DONT do it!
It could be a situation like the one a Vaillant engineer is going to on Saturday where the boiler is fed from one supply and the controls from another.
So, with the boiler isolated the inside still has mains voltages!
If he replaces any water components he will then find the filling valve does not work so he will be unable to repressurise the system. If he has not electroluted himself first that is.
Tony
