Infamy, they’ve all got it in for me ;-)
If it really is in good shape – no wobbly rear pivot, no rust on the rear triangle, stuff like that – then £300-£500 wouldn’t be unreasonable. It’s a short-whelbase with rubbish brakes and tyres, but someone might want to ride it as is, or upgrade it.
I see quite a few which are that age and completely wrecked, though. Two years in a row, I’ve been donated bikes like that, and I rebuild them and sell them with all profits to MSF, but it’s not really economic to do that.