You can ignore Parkers or any other price guide when it comes to a Williams. In the pictures, it looks in excellent condition and with such a low mileage, i can only assume that its in good nick
As Williams in this sort of condition are getting harder to find, i think your valuation is a "giveaway"
Although its easy for anybody to post on here, it would be much better if you sold it privately. Prices for cars which dont "look" as good as yours are still much higher in places like the Autotrader, but it all depends on how quickly you need the funds, thus the part exchange route
Certainly if £3500 is all your looking for, i think there would be a queue forming to pay that (me included!!)