Hi Bart
From the bank's point of view, they have, in effect, a guarantor, so why should they reduce their security. If you don't pay the mortgage, then they will come after her.
So I don't think that you will get the bank to agree unless you offer them something in exchange. For example, you will pay a lump-sum off the capital or convert to a SVR mortgage. However, I don't think it's worth that.
This affects her more than it affects you, as she has a liability for something which she gets no benefit from.
If you are happy that you will be able to pay the loan over time, then she need never get involved again. Go to your solicitor and draw up a legally binding agreement with her. So when the house returns to positive equity, she will have no claim on it.
Maybe at some later stage, the banks may change their policy, but I doubt it.
Brendan