Just to add a bit of clarity to this. BASSA is not a union. It is a branch of a union, Unite, therefore it escapes a lot of bothersome interference by accountants, Certification office, HMRC etc.
If BASSA make payments of salary to any officials, they would be required to have a PAYE scheme. Whether or not they were a union, or a branch of one, would not matter, only if said salary were paid from their own funds.
However, payments of out of pocket expenses could be made without HMRC involvement, if that was all that was paid.
Generally, any organisation/club which collects subscriptions from members, prepares accounts to present to members showing how their money has been spent. These accounts are usually available to members to approve at an AGM. This has nothing to do with HMRC requirements.
Mutual trading organisations can be expected to make Returns to HMRC in certain circumstances, see this link
HM Revenue & Customs: Unincorporated organisations and Corporation Tax