I've been offered a Giant Talon 2.5, 8 months old for £220 by a guy who's been bitten by the bug and is upgrading.
I was thinking it might be o.k. for the missus, who's been after a bike for light offroading with the nipper and me.

Obviously I'll check it over, but it looks in vgc in the photo I've seen and much better spec than we could buy for that price new.

Not bought 2nd hand before. Reasonable price d'you think? £475 list, iirc.

Soundsa reasonable price but bear in mind that Giant's are heavily discount at the end of model year – so about now.
Should be fine for a starter bike. The forks are reasonable basic so check them over carefully.