I lost my fuel tank filler cap at Silverstone, and whilst I know the inside diameter is 38mm I'm not sure what measurement to take to get the thread right. I've bought a replacement neck plus cap from JJC Race and Rally but whilst it has 38mm inside diameter the thread is completely different . Any ideas what measurements I need to take or where I could look for a replacement?

Cheers! Keith

38 mm sounds a lot like 1.5 inches. There are several thread types it could be - BSP (old British Standard) NPT US imperial pipe sizes or metric. Start looking at Google for thread pitch and diameter

Here are a couple of pics of the capless thread in the hope this might inspire someone to say "!! I know what that is!!!" Any suggestions as the best approach if I can't find the same cap? Get one machined or take the tank out and get the new neck and cap I bought welded in its place?

Take a look at the Compbrake website, they sell that style of filler.