I think that's a good price given by the dealer tbh!
Me too, I can't see them selling that on for more than £16k at absolute maximum (depending on options)and most dealers would be looking to make more than £1800 on a £16k car taking into account your low mileage. Parkers info below on an 09 plate GTI 3 door manual with many more miles than you:-
Original Price £22,415
Franchised Dealer £13,985
Independent Dealer £13,590
Private Good £12,930
Private Poor £11,270
Part Exchange £12,135
The prices above are based on:
A mileage of 40,000 (average for age)
Options selected: None