Think it was flashing the RUU that locked bootloader.
I was just messing with the phone last week while bored, decided to Supercid, then flash an RUU, can't remember which because i have a new laptop and have downgraded from Win 8 to Win 7 on it so have no files left that i originally used, then changed cid to one of the ones that the OTA's were available on (before UK version was released), then back to Cid htc__001.
Didn't really notice that the bootloader had locked again until i booted to it to check the Hboot version after OTA'ing, so can't 100% say at which point the bootloader was locked again.
Just messing on stock at the moment, been busy at work etc, but it is only a matter of time before i obtain root again, just thought it would be easy option to downgrade firmware and unlock bootloader again using revone.
HTC One Silver