Thanks Clive, but luckily, it turned out to just be a 'cam pully' (
), so not as serious as I first feared. Thanks for the link, though, that's firmly bookmarked. The car gets serviced every six months or so, so next time, I will get them to do a more thorough check over.
Re the new speedo, I was thinking it was a bit suspect when I was considering buying it, but a Google search revealed that it was quite common for speedos to go on the 806 models, and the guy was up front about it, so I thought it worth taking a chance, as the car itself is in really good nick.