Can't recall who recommended these Basin Mate things on the group, but thank you;
I ordered one to try, & used it today on a right bugger of a job - it worked a treat. Four attempts to tighten & seal the poxy Wickes waste & loads of LSX failed - this sorted it first time.
Just ordered half a dozen.
And someone was asking about the taps that come with flexible connectors, with swivel nut connectors. The worry was that they wouldn't seal on a service valve.
These seem designed for the job (although I've never had a problem);
So I've ordered some of those as well.