Birmingham Midshires increased my fixed rate mortgage payment?

I have been with Birmingham Midshires since 2017 under an interest-only 5 year fixed rate buy to let mortgage. They have recently written to their customers informing them that they have migrated onto Lloyds Bank systems (who own BM) and as a result, changed the dates they calculate mortgage payments which then created for me an additional £486 of additional interest to pay.

They are now taking this sum via increased direct debit payments from our account. When we complained, and I understand many of their customers have done so for the same reason, they cannot logically explain why we should be paying more money under our fixed mortgage just because they decided to go on a different system.

They have issued what looks like a standard response letter which does not clarify the position at all, and indeed they argue that they are entitled to make changes to the payment!

We are now going to the Ombudsman amongst other potential agencies and I wondered if anyone else on this forum has been impacted by this and what they are planning to do about it.

All comments would be appreciated.

Eren