Andy Hall wrote:

Yes. Plumber's merchants have them to temporarily blank off
compression fittings. You need a flat rubber washer in the cap to
seal it, or you can wrap a few turns of PTFE tape around the male part
of the valve.

Okay, thanks for that - I though they must exist however have been
surprised to not see them in the likes of SF, TS and to a lesser extent
B&Q. Just goes to show I suppose...

Will call into a plumbers merchant and pick up a couple.. failing that
I'll do what Gav suggested and rustle something up.

Mathew