I feel it's my duty to provide the new sorry conclusion to this.
A couple of days ago (3 months after buying), I saw the message in the bottom right of my desktop "Windows 7 Build 7600 - This copy of Windows is not genuine". It hasn't locked me out, but the desktop's gone black, and I can't receive normal Windows updates.
After contacting microsoft, they said I have a 'volume license' version which is meant as a discount for businesses etc.
I'm not going to name the seller, as he has given me a prompt apology and refund (apparently, he got it from another source, thinking it was genuine).
Moral of the story, be very careful about going for these "somewhat-cheaper-but-still-expensive versions" (apx. £160 -> £105). I'll probably now need to rebuy, and then format my HD, and reinstall all over again (thankfully, I have most of my data on a 2nd HD, and mostly Windows stuff on the main HD - I would also advise others to keep the Windows stuff on a separate small 60-100GB SSD for speed and to allow easier recovery if one of the drives should die).
And you bet the version I will buy this time will cost about £160 give or take a few pounds.
I think what also annoys me is that Microsoft didn't say this was a 'volume license' version when I first installed Win7. That would have made this conclusion a lot quicker. It would be so easy for MS to add this simple message.