Great - thanks for the advice. I have to admit that I hadn't considered the possibility that it was one of the other partitions screwing things up.
I'm at work now, but I'll try booting into the Recovery Partition later to see if that still works. In terms of creating Recovery discs etc, I'm not worried about not having them and never in fact bothered creating them. I have a Vista Install DVD supplied by Dell that came with the PC for using to recover / reinstall from as well as the Win 7 Install DVD.
I've downloaded the Partition Iso and burnt to CD. Will boot from that later and get a screenshot of the Partitions on the disc. If it comes to it, I'm not worried about keeping the OEM and Recovery Partitions and would be happy to delete them to just leave the Vista and Win 7 partitions or even just the Vista partition on there as Primary partitions as long as I don't lose the data from the Vista Partition.
If it comes down to it, I would even be prepared to protect the data by buying a new HDD, removing the old one and performing a clean Win 7 install onto it from which I'd then recover the data I wanted to keep from the old HDD.