I am making an assumption, that all benefits are calculated in a similar way when it comes to the “sliding scale” (what you refer to as tariff income).
The sliding scale means that for every £ over the bottom level ( in this case £14,250), the amount paid by the LA is reduced, so the amount the LA wouId pay when you have £23,000 is much lower than the amount they will pay when you have £14,500
In theory, you should have been told to let them know as the capital reduced, so the LA could up their payments, as the capital reduced. Somehow, I doubt that was mentioned.
If you have kept them in the loop, as the capital has reduced & they have increased their payments, then the increase as your mums capital drops below the threshold wouId be small, but at least she wouId then keep the little that’s left
If no interim assessment has been made, then the LA has only contributed as if she had the higher capital, so their increase could be considerable
If you go ahead & request a reassessment (which they are supposed to do annually), find the last assessment letter & all the paperwork you had for that. Then add any & all paperwork for each section from then to date.
So, bank statements for each bank account. Notification of any increases in pensions, AA, council tax, stocks, shares etc
Hopefully you have kept a record of how the reduction in capital has been spent. Care home fee contributions, hairdresser, chiropodist etc, put all those bills in the file, then try to be very calm with them
Lots of luck & hopefully you will get an assessor who is human