Thanks all for the comments. Responses to some of the points raised:
- Yep, its a fishy type smell. I had the same smell years ago in a bathroom elsewhere and it turned out to be a very old light fitting.
- The whole lot is going to be replaced in about a years time and the rest of the boiler / airing cupboard is so old that aesthetics arent a consideration. Im an average DIYer so its a case of doing it safely / not driving a coach & horses through regs.
- The immersion heater & tank were replaced about 10 years ago. The switch itself is much older, probably at least 40 years old. Cabling into the switch from elsewhere is original. Cable from the switch to the immersion heater is 10 years old.
- The heater itself is 3kw and the circuit is 20A
- Ive put a picture of the switch and the consumer unit he https://bit.ly/3vx3j4S
- My thinking is to surface mount the new switch on the wall to one side. The length of the cables wont be an issue.
- Ive enough slack to take the 10cms or so closest to the switch of the original cable off.