On 28/12/2016 16:46, DerbyBorn wrote:
Is an immersion heater one of those situations where - as it is the washer
and the faces that perform the function of sealing, it is totally un-
necesary to apply any sealant to the threads. (I wouldn't - I would just
ensure the faces were free of debris and damage - and the washer was good.)

I think that is a good point.

Possibly I was misleading and the jointing compound was applied to the
washer.

The first time I refitted the immersion I didn't use jointing compound
and it leaked. So I put jointing compound all around it the next time. I
bought a better spanner too, so I can't be quite sure what fixed it.