If i was to phone bt and ask for a new number would there be a charge, and what would i have to do with aol broadband ?

A change of number is done for free.

It is a good idea to let your ISP know the number their service is on has changed.

I have recently changed my BT number and there was no charge. When I asked if I should let my ISP know the advisor said no need. My advice would be to check with BT.