Thanks for the advice guys, I've ordered a new set of wheel bolts off Ebay.
For piece of mind if nothing else, every time I hear a rattle while driving I'm convinced my wheels are falling off! Been checking them every day with the torque wrench and they haven't loosened up at all, but don't want to take the risk.
The thread length of the new wheel bolts I've ordered is listed as 28 mm long, and I've measured my old ones length at 24 mm, so hopefully the extra 4 mm will give me the extra turns I need.
Derek - I'm in Staines, so not that far from Slough, if there's any problem with this set I'll take you up on your generous offer.