VintageKrug gave a link to the "Trade Union and Labour relations Consolidation Act 1992" which contains these words
A member of a trade union has a right to request access to any accounting records of the union which are available for inspection
So I checked the reply that
BASSAwitch had received and it contains these words
We have been told by the branch secretary that the accounts you are seeking are not available at this time. If at any time in the future they do become available for members and ex members to view, we will endeavour to make that known to you.
The bold type is by me to bring attention to the word "available".
IMO this means that the "28 days" is not something that will be met.
I apologise if I am the bringer of disappointing information.