I just bought an old saab which is only 11 out numerically on the plate from one my parents had while i was younger (this was one of the things that made me buy it! - they probably sat together on the forecourt!). I still have the number plates from his old one (a souvenir!), and it looks like both cars came from the same saab dealer, who presumably bought a big block of numbers to put on the new cars. Ive just spent ages looking up 10 or 15 either side of them, and not found any to still be about apart from the one i have now :(
It doesnt seem to want to give me any details on my landy ('69 H reg) or my dads old morris minor either (an XX NNNN plate), so its not a totally complete system, which is a shame.