My monthly payment amount

Hoenir

Halifax may well have estimated the interest for the period 15th to 31st August , but excluded the payment due on the 1st September in their calculations. Simply a mathematical quirk of providing instant information part through a calender month when there's been a rate change. As and when the calculation is formally done. It will naturally self correct to a lower amount. 

Personally I'd take a two year fix and overpay. Rather than sit on such a high SVR. Even with ERC's if you don't port the mortgage. There's going to be a saving.

Also use some of your savings put aside for the move to pay the mortgage down. As you can always reborrow again in the future.