2001 Symbol carver leaking

Hi, looking for advice. I filled the water tank up,after winter drain down, and the drain tap won’t reseal so off I go and get a new o ring, still leaks! Also it seems to also be leaking at the bottom of the heater and feels damp round it inside. Didn’t realise there’s an electrical isolater inside so might have had the power on with an empty tank. If so what damage could that do?
I read that there’s a bypass insert to allow to run only on cold water, any ideas where I might get one so I can have a cold water whilst I sort the carver out?

If you mean a Carver Cascade, I had a similar problem on mine and it turned out to be a very fine crack in the centre of the drain plug.

IIRC there is an overheat cutout in case the heater is turned on without any water in it. I believe there is also a meltable plug in some versions, but I ~think~ that should only be a backup, not the primary overheat protection.