You probably need to decide how likely you are to trade up. If there is the slightest likihood, hold onto the cash & continue to save. There is no guarentee they will allow you to transfer your tracker mortgage over as it will be a new loan - but you'll have a stronger bargaining position if the mortgage is still in place.
Having the cash at that stage (if you buy) means you can pay a large deposit & keep your new mortgage as low as possible (as it will be a more expensive loan if you end up on variable).
Best thing is not to make any rash decisions - once you give the bank the cash, you can't get it back, so be sure you don't want it for anythng else in the foreseeable future. So get the best return you can for the next month or so while you make your decision.