There is the usual array of Glass and Parkers for example; some offer a free valuation, but most require a registration and a fee to get the best out of them; i.e. open up all options etc... to fine tune the valuation.
I have always tended to look at the used car pages; the likes of Autotrader, eBay and Gumtree and make my own mind up based on what is comparable and what is being asked for across the range. Cars like the A2 can buck valuation guides and the 'for sale' section; personal valuation can be the decision maker in these cases (i.e. heart over the head).
Good luck with the viewing.