It seems correct to me. Each month there is an interest charge of close to 0.23% (one twelfth of 2.75%). So in month one the interest charge is £3208. You pay £9500 a month in payments, so by the end of the month the amount to be repaid is 1,400,000 + 3208 - 9500= £1393708.
So next month the interest charge is based on a smaller outstanding balance, and you continue to pay £9500 so by month 3 your balance has fallen again. As time goes by your outstanding balance falls by more each month so the interest due falls and you continue to pay £9500. By the end of 15 years your balance is nil.