A mac key lasts for 30 days and you can use it at any time in that period. If you dont use the MAC key then it expires and you can request another from your ISP.
Strictly speaking an ISP shouldnt process an order without a MAC key - although talktalk/aol (FMPF) will do so.
>> so if I ask BT for one on say 2nd September and then give it to O2,will they/can they start my new BB contract before the existing contract(30 sept) ends.
Yes.
Your existing ISP must provide a MAC key within 5 working days of receipt of your request.
Once you have your MAC, give it to O2 who will then be able to process your order.
Be/02 will then give you your migration date... which normally is one week later.. although it can be a day or so either side.
So if you apply on the 2nd - BT have to give you a MAC by the 9th.
If you place your order with o2 on the 9th.. then migration should occur sometime around the 16th.
Good luck