Very strange! I was indeed careful and took out my sim card, did all my backups, gold card, rooted, then ran out of time, thought I'd put my sim back in so I could use the phone and it appeared to be sim locked, put my card in a spare phone to make a call then back in to my desire, still appeared to be locked. Then I put this post on here, waited for a response (thank you) and was about to try your suggestion but tried the sim again first and it's working now!
A bit confused but hey ho, it's off to flash we go, fingers crossed again.