When the shower is on the light turns off and the water runs cold. When the shower is switched off, the light turns on and I can not turn it off.

Is this a ceiling mounted heat lamp or what? I am thinking that maybe this is a pilot light (electrical light to show something is powered) and you have an electric water heater. More details please

It is the light on the isolating switch (pull cord) for my electrical shower. I know I need the light to be on if I want hot water from the shower and it forms some part of the circuit for the electricity. Does this help at all - sorry I really don't know anything about the workings of a shower!

Glad you explained and sorry but I have to back out of this. Absolutely no experience with electric showers, are you in the UK?

Yes in the UK. Thank-you for having a look anyway!

We don't see them much on this side of the pond. Have agood day

YUP! No experience with electric showers here... ;)

I'd bet that we'll be seeing them soon enough!

Sorry...

Mark