I have phoned OSCR technical helpline and their response has been copy pasted below-
There are two types of accounts which can be prepared by charities, Receipts and Payments accounts and Fully accrued accounts.
Where receipts and payments accounts are prepared as is the case for the XXXXXX then these can be examined by someone who the trustees consider has the necessary skills and knowledge to do so. This does not need to be a member of a particular association
If the Trust has to prepare fully accrued accounts as it is also a company limited by guarantee. This means it has to be examined by someone with one of the qualifications recognised in the legislation. A list of the relevant qualifications can be found in section 3.5 of our guidance which can be found here.
If the chartered accountant is not a member of the organisations listed then they cannot sign off the report.
The requirements for Independent Examination are set out within the Charities Accounts (Scotland) Regulations 2006 (as amended). Within schedule 11(2) of the Regulations there is a list of the various bodies of which an Independent Examiner can be a member of to examine fully accrued accounts (as provided in my previous e mail). Unfortunately a member of the other bodies cannot examine a set of fully accrued accounts.
I hope this clarifies the position; however should you require anything further please do not hesitate to contact me. "