I was given a normal tel. number 01.. by one of the guys in the company which I used last week and it worked fine.  Now I see that this number has been blocked (by BT I presume).

Warning:  This could be the new trend  :o

I think you are going to see more of the same problem.

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.