On 27/04/2020 18:56, John wrote:
If only people would realise that it is the gasket that created the seal -
the tread it sher to pull the faces together - and what about a smear of
coppaslip?

Indeed. I'm not a plumbing expert, but that's how I understand the
situation.
The immersion manufacturers that I've read installation instructions for
say you should only need to use the supplied gasket; no thread sealer is
needed. One did suggest using PTFE as a lubricant.