In message .com, TC
writes
Apparently its M5 x 40mm - I assume the threads are always the same.
Any recommendations for somewhere to get something strong on the web?

A single bolt via the web doesn't sound like it makes much sense unless
you are out in the sticks somewhat.

a local bike shop really is the obvious place - note that the heads on
seat post clamp bolts are a sort of lower profile than your average
Allen headed bolt, and some have shoulders on the body of the bolt etc.

St John Street cycles are good for odd bits/spares, but aren't cheap,
and have (or did anyway) a fairly hefty minimum postage charge

http://www.sjscycles.com/
--
Chris French