I know this is an outrage and I should be hung drawn and quartered for
even contemplating it but needs must (!)

I have an old 6-foot single radiator in the bathroom which is beyond the
end of its life and showing (since yesterday) a small rust hole/leak. I
have a 1m double that would do the job in its place. I'm planning to
renovate the room comprehensively later this year but currently have
very little time available to do any major surgery on the CH system.

What I would like to do is join some copper or plastic pipe to the old
valves then mount the new radiator to this. This will save me draining
it all down, lifting the floor and re-routing the pipework for the
moment. It will also, of course, look dreadful but it's a temporary
measure so I'm not worried about that.

I know you can get extending tails, but they wouldn't stretch this far,
so the question is, what do parts I need to join the 'radiator' side of
a valve (TRV and LS) to 15mm copper tube? I'm imagining it will be
something like a 3/4 tap connector but I'd rather make sure I have the
right thing before I get up to my eyeballs in the job!

Normally I would just go and chat with my local PM but I went round
there this morning and to my horror they have closed down, so I need to
go and cultivate a new one somewhere.....

Cheers chaps