Firstly, don't increase your bid - what's the point of bidding against yourself?
Contact the EA by phone and in writing (by fax) stating that you want the sale agreed before say 5pm on the 16 December and that if you don't hear from them by then, you are withdrawing your bid.
They've had long enough already, and the EA knows damn well that Xmas is a slooooow period for house sales - if you put a bit of pressure on, saying how good it would be for all involved to have a firm agreement before Xmas, it might just force them to agree. Knowing the EA though he'll come back saying 'oh, another grand and they'll deffo sell to you before Xmas.'
Try the ultimatum though - it could work, and remember, there are other houses out there! Good luck.