On 2020-09-29 18:27:29 +0000, ARW said:

I did this last year in a hard water area. There was a massive bang and
then the sound of lumps of limescale dropping down the inside of the
cylinder. Probably a 30 year old element that had never had a volt
across it.

Arrgh - the cylinder was installed about 17 years ago and in London
it's hard water. Maybe this is not such a good idea...