On 03/05/2021 14:04, Andrew wrote:

If it's a standard 3kw immersion element then simply checking the
tightness of the connections inside the existing switch may be all
that is needed.

Except that the OP described it as "a thin-ish one, mounted on the
wooden door frame" which sounds very much like a light switch which
would only be good for 6 amps!