You also need to read sections 28 and 29 which specify which records are required to be kept.
The "which are available" refers to records older than 6 years which are not required to be kept.
29 Duty to keep records available for inspection.
(1)A trade union shall keep available for inspection from their creation until the end of the period of six years beginning with the 1st January following the end of the period to which they relate such of the records of the union, or of any branch or section of the union, as are, or purport to be, records required to be kept by the union under section 28.
This does not apply to records relating to periods before 1st January 1988.
(2)In section 30 (right of member to access to accounting records)—
(a)references to a union’s accounting records are to any such records as are mentioned in subsection (1) above, and
(b)references to records available for inspection are to records which the union is required by that subsection to keep available for inspection.
(3)The expiry of the period mentioned in subsection (1) above does not affect the duty of a trade union to comply with a request for access made under section 30 before the end of that period.