Ooo err...I've just overheard my son's conversation with his pal, the vendor. He said something about having noticed just how much the 800 is fetching on the second-hand market. I just have to offer him the chance to back out of the deal. One wait's with bated breath.
Thanks for the links, I'll be armed with a bit more information before seeing the guy this PM.
The 28 just will not work in TTL mode, so I'll pass on that one as well if I feel morally obliged to back off the other deal. It will be a very short flirtation with flash photography.
I could build an interface for the old one, using a voltage-capping diode to protect my camera. But, it would go on a list of jobs to do in this lifetime.