Rip their hand of at that price pastille, i bought a brand new 11th Edition not 12 months ago and it was 40 quid because my 1993 version was literally dropping to bits.
I do know that the 12th Edition is out from liaising with AB over the Joint Effort and he has just purchased one, that's maybe why the're selling them off for a tenner.
If you do buy one, a tip is, keep the outer cover on it and seelotape it down cus the binding doesn't last at all on these books if it's without a cover. The spine part of the hard cardboard cover on mine has already disappeared.