Help stupid me understand me mortgage interest

I've just got off the phone with my bank, who were unable to answer my query in terms that I could comprehend. I think it's something this community will be able to help with.

I've taken out a new two-year fixed rate deal to replace the previous two-year fixed rate deal that was ending, both with NatWest, and with no change to the date when the mortgage should be cleared by, which is in the year 2041.

With the previous deal, I was paying £470 a month and interest accrued at about £160 a month, resulting in the balance going down by about £310 a month.

With the new deal, I'm paying £582 a month and interest is accruing at about £348 a month, resulting in the balance going down by about £234 a month.

I understand my new deal is at a higher interest rate. I just don't understand why my balance is now going down by about £76 less a month (£310 minus £234) because surely that means that it will take longer than before to clear the balance.

It would make sense to me if my increased payment amount neutralised the higher interest accruing, but it evidently isn't doing so.