Thanks very much for all your replies and advice.
So, on Friday afternoon after work we went to view the bike and ended up buying it for £750.
It is in immaculate conditions. No damage to the paintwork (actually the cable routing is better than the one on my Giant TCR) and no signs of cable rubbing at all. Cassette and chainrings look barely used, even the tyres are original and with little sign of usage. They seller said he did less than 500 miles on it, most of it on rollers and judging by the conditions I can easily believe it.
I tried the bike myself, the 105 is super crisp and the tektro rim brakes seemed to be working fine too at a reasonable speed (and I'm 50 pounds heavier than the missus). Size is alright although I'll be buying 2 shorter stems and I'll do some experiments to see which one gives her the best position.
For now the missus is buzzing and can't wait to use it. Let's see how long before it goes in the garage to collect dust