On Monday, June 25, 2012 7:21:57 PM UTC+1, Justin C wrote:

Thanks for the reply, you've made me double check my
measurements. It's not easy when they've suffered crevice
corrosion (the leak was fixed last year, but I didn't
replace the bolts at the time).

The digital callipers say they're 0.3715"... or about
9.43mm. That's not M9, nor is it 3/8" which should be 0.375
decimal inches (decimal inches?! the mind boggles). I'm
guessing that 3/8 is what they are and either the callipers
are out or these were cheap bolts.

Actually that sounds pretty close to me - I think the tolerance on the major diameter is about .007" for medium fit (which is the tightest tolerance recommended for bolts under 3/4"). That means the diameter could be between 0.375 and 0.367 inches.