Try the car buying services such as We Buy Any Car (WBAC), Wizzle and Motorway. With any of these car buying services, always be completely honest on condition and declare any defects (e.g. scratches, parking signs etc.) to avoid any initial indicative online offer being knocked down when the car’s viewed.
WBAC have a reputation of making lowball offers so you could use that as an indication of the minimum you’d be likely to get. There are forum members on various forums who’ve used Wizzle and Motorway who have provided positive feedback on the selling experience and the amount they were able to sell their cars for.
It’s also worth getting a price from local VW dealers. With new cars being generally in short supply, demand for good late date used cars has been high so many dealers are short of stock, so you may be pleasantly surprised by what a VW dealer would offer.
It’s also worth checking to see what GTD’s of a similar age / spec / mileage are being sold for by dealers on Autotrader and in the used cars section of VW’s website. Dealers’ selling prices probably include a mark-up of around 20% on what they pay to buy in cars; that mark up covering such things as the cost of preparing for resale, providing a warranty, dealership overheads, dealer’s profit margin etc. So if you reduce the retail prices of cars being offered for sale to remove the dealer mark-up, it’ll give you a rough idea of what you might be offered if selling to a dealer.
Good luck with the sale!