Thanks for those details. I've been doing some scribbling (it's been a quiet night-shift so far for me here in Australia) and realised that my long held belief that (when considering the operation of an RCD) a rectifier output is isolated from the input just like that of a transformer is so much kangaroo poo. On reflection I can't think where I got that idea which is all the more worrying, I obviously didn't draw any diagrams to prove it.

Well I did say that my advice wasn't worth the cup of tea Paul would have offered me if I'd visited him in Hampshire

Kit