I solved the problem by selecting all of the clips in the timeline together and setting them as the loop region, and then selecting 'render loop region only'.

This is difficult to explain, but I suspect that maybe Vegas was screwing up because when I was originally organising the order of the clips I had temporarily put clips much further down the time line (i.e. 7/8 mins) and after I removed these clips Vegas still seemed to think that the project was 8 mins long, despite the fact that there was nothing on the timeline after 5 mins.

Anyway, problem solved.
Thanks guys.