Ok, mystery solved, I feel really stupid!!!
Tried again, and the third from last character looked like O or 0 - I tried entering O or 0 earlier in the number with the same error message, so thought obviously they don't allow a O or 0 in the product key, then tried a Q, which has worked!!! In my defence the last few characters of the number have worn off a bit and it was virtually impossible to see the tail of the Q!!! That's my excuse and I'm sticking to it!!!
So problem solved, thanks for the help though!