Increasing term issue

I spoke to my bank and asked if I could overpay my mortgage each month temporarily and not on a permanent basis.

They sent me out a form. I signed the form but added to the form that this is not on a permanent basis. Heard nothing.

I now want to decrease the payments to the 'normal amount'. They say, apparently, I'd signed a form to decrease my mortgage term so have to continue paying these higher amounts.

If I want to now increase the term, I will need to have a telephone conversation with someone for up to 2 hrs and fill out a budget planner.

The issue isnt about whether I can pay or not, the issue is I want to stop overpaying as I thought was possible.

I see it as their mistake, but I have to go through some bureacratic process of them assessing my income to increase the term. I'm guessing if they see I can pay, they won't allow the increase in term ?

What can I do, if anything ?

Thanks