I believe that the period within which the union had to produce the accounts when in receipt of a request from a member was 28 days. The union then has 28 days to produce said documents - such is the law. They may well have these things easily to hand, they may not. That is not really relevant, even if they are easily available they are under no obligation, legal or moral, to produce these documents in period less than 28 days. No inference be drawn from them taking the full 28 days to produce these documents.
I despise the cancer that is BASSA, but they have the right to the 28 days. If there is something dodgy going on, doubtless all will be revealed but in due course. However, it is quite possible that the books have been kept immaculately and all is above board. Either way, the union would still be entitled to the full 28 days if they wish it.