I was recently offered £9.5k for mine as a trade in 05 plate, 55k, 5 door with leather and sat nav + full vwsh. Admittedly I think they were very keen to sell me the other car but I do see similar vintage cars as my own on the forecourts for over £10k. I bought it nearly three years ago with 30k less miles for £13.5.
I got the impression dealers are quite keen to keep these in the VW network to keep residuals high, probably because there are so many about on PCP deals and they don't want their gfv's to be eroded to the point the returning customer cant afford to trade up or even worse they have to take the hit on the residual value.
In any case, VW's generally have solid residuals vs the equivalent Ford / Vauxhall so I wouldn't worry too much about it.