Thank you, thank you, thank you! Not only could I buy from fictionwise (no 'wrong country' there...) but I saved myself a few quid as well, and just in time for 4 hours of train journey tomorrow - result!
It's been a while since I used Mobipocket, and I'd completely forgotten I could buy mobi books from other sites, so thank you again everyone, very much appreciated.
Cheers, Pete.