Valuation advice

[wheres_my_beard]

I'm thinking of selling my 18 month old Whyte Kings Cross to help fund a new bike, and wandered if anyone could give me an idea of its rough value. The RRP is £800, but it has upgraded brakes (Avid bb7's with new pads) and Jagwire cables, and would be sold with 4 sets of tyres (Continental Gatorskin slicks, Specialized Borough XC semi-slicks, Schwalbe CycloX King CX tyres, and Schwalbe Marathon Winter studded winter tyres worth £200+ together). It's done a lot of miles, but is in good working order with only about 400km on the new cassette and chain, and would come with some M540 spd pedals.

Any ideas?