"would planning permission need to be cleared up (if it is needed at all) before the sale can complete between family members?"
If she is paying you from her own funds, she can do what she likes. The problem, if it is a problem, becomes hers alone. Mind you, it will remain a problem down the line for her.
If she needs to raise a mortgage to buy you out, the solicitor, if aware of the issue, ( and they should be made aware of the issue) will be qualifying their Certificate of Title, so the bank will require that the matter is investigated and resolved.
mf