I'm guessing you guys did yours before the changes to registration to Swansea took place?
I may try a letter to the Chelmsford office otherwise I'm going with one of my own.
The chances of picking 17 random digits using 33 characters for each position and matching one on the system already are 1 in 400,000,000,000,000,000,000,000,000,000,000,000,00 0,000 so I should be OK to make one up based on a mix of my name, date of birth, phone number etc.
Cheers
Stot