went to straighten my seat belt and noticed that the trianlgular support (what is proper word?) was not moving. Tunred out bolt had undone a tiny amount and wastrapping the thing.

SO CHECK YOUR SEAT BELT BOLTS!!
anyway, so that this won't happen again, refitted all the bolts with some studlock.

Do you mean the triangular metal belt end that the bolt goes through? And should it be moving? When I bolt mine down they're trapped by the bolt head.

Good practice seems to be they should swivel, but some manufacturers are being quoted as saying theirs don't need to.

See this thread and the other thread I link to in it towards the bottom.

http://www.locostbuilders.co.uk/viewthread.php?tid=87419