On Wednesday, 31 January 2018 08:24:18 UTC, jim wrote:
I'm going off the 2.5mm2 feed idea, wouldn't that need fusing down
to be safe?

Probably ok, the 3kW immersion heater load is fixed and won't overload, and short circuit protection should still be adequate by the circuit fuse. (2.5mm is fine as a spur on a 32A MCB ring circuit).

Owain