I'm not au fait with the exact ins and outs of the mortgage draw down process but I would have thought that once you dip into the cash then effectively the whole lot is yours and you start getting charged interest from that point, even if you don't actually draw down the rest and start making repayments, for another couple of months.
On a 300K mortgage @ 4.25% that would work out at over 1K a month in extra interest payments for the benefit of a slightly lower rate for a 2-5 year period of your mortgage. It's debatable whether you would save enough over that term to justify the expense.