I phoned Microsoft when I got the reply here. The first time I did it though my phone ran out of credit after waiting to be answered for 7 minutes. I tried to email them but when I entered my product code I got the following message:
The product ID number you entered is not supported in the country that you have selected. Enter another product ID number or select a different support option below.
Which didn't make much sense seeing as I'd selected UK and I bought the product from amazon.co.uk.
I just phoned them again and I'm glad I did. That must have been THE most helpful no nonsense helpline I've ever used. I was cut off mid conversation and was rung back immediately too. I'm waiting for some parts for my new build so they are going to ring me back when they arrive!.
Thanks for suggesting I ring. It's not sorted out yet but I'm very optimistic that it will be.