How many bikes have you got already?
Vintage bike prices appear to be still down a bit, but we're getting into the Spring so may be worth it. Retrobike / eBay are probably your best bet - value-wise it'll depend how original it is. Is the frame re-sprayed? If so it's worth less I'm afraid. I have the same issue with mine.
Think it's original paintwork it's got a few marks but nothing really bad or dents.
If it's a Special Products Division, and the frame number confirms this, then I'd suggest the frame is worth £250-300. I can't quite see the groupset but the value is in the frame imo.
eBay as an auction will get you (probably) the best price, but it's a risk that it only goes for the start price. Retrobike will get a smaller level of interest, but they know what they're looking for specifically.
I try to avoid Retrobike for the sake of my bank balance and domestic harmony.