Thanks chaps. I guess we are at that difficult point where the cars are still young and so close to the new price, which means people may decide to just spend a bit more and buy a new one. The other problem is that it will probably be dealers that show the most interest, but they are professional buyers who will always drive a hard bargain - offering trade or a bit more for a part exchange.
The frustrating part is that as the cars are nearly new, assuming no crash history, the cars are absolutely fine to buy privately, but, as they are also relatively new, people would pay three or four thousand pounds more for the same car from a main dealer!
I'm going to take my chances and advertise in autotrader. Main dealers will always want a 1 year old GTI in good condition, so I'll just use them as a last resort - which is fair enough given the prices they are prepared to offer assume you are desparate to sell!