It sounds about right to me. Think about it - your mortgage has slightly more than doubled and is at a higher interest rate, therefore your interest payments are notably higher. On a back-of-a-virtual-envelope basis, it sounds like the mortgage interest relief in each case is pretty much on the button (I know it's in the right region for your first mortgage, having had a mortgage of similar level over the same period).
However, given that rates may have varied, that different institutions may have slightly different calculation algorithms, etcetera, the only way to be certain is to check your mortgage statements and check that the mortgage interest relief in each month is one fifth of the interest payable, subject to the caps ClubMan has already set out.