hard to put a value on it really, I feel Â£50 might be a bit mean but if thats all that person wants to pay - thats what its worth , to them anyways.
the frame is the thing folks would be most interested in as the groupset is not really desirable and quite.....low on the shimano scale. (yes, retrobikers can be snobbish)
your best bet would be to auction it off on ebay, if you don't want to sell it too cheap put a reserve on it and hope someone wants it as much as you did when you bought it - if you can't sell it, keep it & ride it.
to give you an idea though -
one on ebay at the mo - Â£60 - no bids - 1 day left
one sold last week ont bay - Â£50 - 1 bidder.
hope it helps
Stabled = 91 Browning Test unit - Trimble Double U - kestrel MXZ Black - 87 Chas Roberts Snow Leopard - 88 Overbury's pioneer - Dave Yates Diabolo - Proflex RF-1 team replica - Mk1 Thorn Nomad - Dave hinde disc - frontiera 531 - Serotta Proflex