I'd have a nose on Autotrader and Gumtree to see what similar are going for.
I noted the following -
2010/105,000 £1990 private seller
2012/111,000 £2550 private seller
2013/140,000 £2495 garage
2013/95,000 £4250 garage
I dunno, £2500 give or take £500 due to condition?
Then adjust for the fact you will be selling privately or trading in. Also you might get a bit more selling to a window cleaner as it will be partly set up. Selling to a non WC you might want to remove the tank and put back to standard and sell or transfer the tank?