I am £150 in credit with BT, my next three months are paid for ( since you are obliged to pay in advance) and BT will not give my overpayment back.
However has anyone managed to get their overpayment back from BT

A little more info is required.
How come you are in credit? How did you manage to overpay them?
Is this actually a deposit that BT are holding, rather than your account being in credit?

They might do, without you asking to, if you're on a direct debit scheme. A few years ago we were in credit, were paying more each month than we were using (hubby moved in with me so they were considering previous charges before we lived together - no need to use the phone so much once we were in the same house). So one Christmas they gave us back £80 and reduced our monthly payments. Not sure if they still do this though (this was 2002).

I have just had excess credit paid back from the water company and from British Gas with no problems. I don't tend to get in this situation with BT as I make very few (if any) chargeable calls using BT so my DD is pretty much the line rental plus a little bit.