Dave Baker wrote:

Wash your bloody mouth out son! UNC - Unified National Coarse -
British through and through. Nothing to do with yanks. You mean 3/8"
I presume not 3/6". Go to a nut and bolt stockist same as all the
other bolt queries this group seems to produce.

Yes, I meant 3/8", it is very hot, you know :-)

I thought hey, nuts and bolts = Screwfix, but no - metric only (or am I
missing something?)

At least you didn't say "Google is your friend".

Cheers
Tim