I contacted 'Tech Support' on 0800 328 1919 and they wanted the Account ref from your last bill and the IMEA number from the phone. Check that the original box and number inside the phone under the battery are the same before you ring them. It took about 30 seconds to give me the 6 digit number to key in. I was asked what SIM I intended to use first (did not know that made any difference!) but I said Orange PAYG. I inserted the SIM, keyed in the 6 digits and hey presto - it is unlocked. I kept the number in case it was required when doing a factory reset but it was not required.
I also understand that the Unlock code is specific for your IMEA reference.