BT could not block the 01 number. That can only be done by the incoming line customer.
This is what may have happened in this instance.....
0845 numbers are virtual numbers that call forward to an 01 number. If you had the correct 01 number then it cannot be blocked as the 0845 number would then not work either. Probably the 01 number you had was certainly calling the correct business, but ended up on some poor sods extension....which has subsequently had to be taken out of service and renumbered.
Hope this info helps.