Have to agree with whats been said before. The main sticking point in my head is that there is an invisible force that stops owners going below £10k. I have a 53 Zed with 63k miles on it, and I know its worth squat now, but knowing that I wont sell it. Even though I want a 370z, I'm having to keep the 350z as its not worth selling it. I think we had a time where people were trying to get out before the prices crashed which is why there were loads going cheap, but now thats dried up a bit more, prices are stabalising as those that are left, dont want to sell. Its your typical supply and demand model. Dealers selling them for £20k were few and far between from what I saw and it was just to get rid of stock they couldnt shift before the 370z lands.
Keep trying and you will get lucky, someone in these unfortunate times will need to sell quick and the price will reflect that. Most now however dont appear to be in a rush to let them go though.