I've just been reading this
Trade union rules and membership register : Directgov - Employment
Two sections which are really pertinent are
Membership register
Trade unions must keep an accurate and up-to-date register of their members’ names and addresses. You are entitled, on request, to:
find out from the register, for free, whether there is an entry about you
be supplied, for free or for a reasonable charge, with a copy of any entry on the register about you
Disqualification from holding office
It is a criminal offence for a trade union to fail to supply copies of the trade union rules when asked.
If a trade union fails to do this, they must make sure the person responsible cannot hold any important elected positions in the trade union for five years (eg member of the executive).
Given that BASSA seem not to know who its members are then they surely must be breaking the rules.
The section on Union elections states that elections must be held every five years at least. There are no grounds given for delaying elections longer than 5 years.