Technically anyone who wants to help can! But yes, also, technically there are no MPs - and when there are, they vary in helpfulness. In some constituencies it's a fair bet that the sitting (non) MP will be returned again, but even so they'll be busy straight after the election, setting offices up again etc. That's a bit long to wait.
I don't know what OP has already said to sfe - so apologies if this is redundant but - maybe something along hte lines of: you asked for proof from the Local Authority that my mother's partner was no longer resident. I have enclosed a letter from them confirming that apart from my mother all residents are disregarded for the purposes of Council Tax. They cannot provide a letter confirming that she receives SPD, as I still live at home for part of the year. However, if her partner was still resident, he would NOT be a disregarded person. What further evidence can we provide?