Hmm, this sort of makes sense now.
I bought 5 2650's from Tesco Mobile which were included in the offer. I also bought a number of 2650's in store, which it now turns out weren't included.
I bought 3 3100's in store, only one of which actually gave the message about the 1000 points posting in the May statement.
Thus, the 5000 + 1000 = 6000 that I've now got. I'm still definately down 2000 for the 3100's, but life's too short :-)
It sounds like they must have done some sort of audit to sort the mess out.
Cheers,
Rick