I’ve a GoPro handle bar bracket in 31.8 size but would like to stick it on an old skinny size bar. I ‘d like to buy a decent shim for the bracket rather than bodge it. I couldn’t find anything decent, can anyone help?

skinny as in 25.4mm or the bar itself at 22mm?

It’ll be the 25.4 standard old school size

I think they do gopro mounts in other sizes, now only 31mm.

However if you go the shim route, a handlebar shim may work, they allow you to use a 25mm bar in a 31mm stem for example.

Problem Solvers make handlebar shims. I don’t know if the have a UK distributor; they’re part of QBP so are available through most US bike shops.

wrap a bit of old inner tube round the bar ? Adjustable, grippy, very reasonably priced and available right now

Problem solvers stuff is available from Ison

sjs cycles on EBay usually do this kind of thing…
Failing that, just search EBay.

inner tube +1