I had one for a while, bought S/H and not quite the right size for me so passed it on.
It's a decent enough frame, without being anything special, that's one of the earlier models, the later ones had thread-less headsets and no lugs. I don't know when they changed, I was told mine was from 1998. I think all Reynolds tubing, the add shows 531 but I thought mine was from one of the lower groups (Could be wrong) Nice 531 forks, IMO nicer than anything Thorn produce today, usual clearance with deep drop calipers, 28mm tyres maximum. I'm pretty sure it would have been UK made, if that sort of thing matters to you.
Thorn sold off the last of the newer ones as straight bar builds, probably with whatever components they had lying about, and ridiculously chap, it's one of my few cycling regrets that I didn't snap one up.
Whether that bike is worth £350 will depend entirely on it's condition, which would have to be pretty good to tempt me. That one doesn't look like it is, bit of surface rust on the frame, the aluminium well weathered, just looks like if it was anyones pride and joy that would have been some time ago.