Did you see
this thread? Andy had the same problem at Virgin Bingo - he had to put some more of his own money in, but he got it out in the end. It's worth calling customer services to discuss it.
It's really disappointing to see Bingo companies pull these can of regulations on regular depositors at their sites. If you won it on the bonus money you got from depositing, it should be yours to withdraw as you see fit.