Replies to this have been interesting - I certainly think that the sub-grand cars are becoming fewer and further between, I think that this is mostly due to a combination of inflation and scrap values have meant that if someone has a car that has say failed an MOT, they would rather get rid to a scrap man than spend time trying to sell it on to the public - plenty of horror stories online about selling stuff on Facebook etc.
Demographics won't be helping either - I grew up in the 80/90's when cars were mostly reliable, but it was understood that cars needed maintenance. The fact that quite a lot of people don't know the difference between a service and an MOT will man that fewer older cars will be serviceable at 15-20 years old.
On the positive side, this does mean that this is probably the best chance that we are going to get to mess about with older cars, so we should make the most of it!