Stalling mortgage application?

I've had a sale agreed on a maisonette on Thursday this week; I've instructed my conveyancing solicitor and the survey is all booked.

I've got my mortgage in principle from Nationwide, but I want to stall my mortgage in proper until March 8th pay day.

Can I do this? Can I delay for longer? I don't want to annoy the EA or the vendor. The house is currently rented so the earliest move in date is June anyway. I know things with mortgages take weeks and weeks, but the longer I can delay my mortgage application, the healthier my bank balance is going to be.

Thanks all money-savers :o