Re: Data Protection Act:
You can formally (in writing) ask any organisation to provide you a copy with all information they hold upon you and they must comply within a defined period (I forget exactly how long). Failure to comply could result in a prosecution/fine.
Organisations are only allowed to record information which is pertinent to your relationship with them. For example it would be deemed inappropriate for your employer to record your membership of political organisations. A grey area forms when you leave an organisation, the case can be easily made to suggest why it would be a good idea for organisations to maintain details of past members purely for their own internal use, e.g. recording the historical collection of subscriptions.
I'm not sure what laws govern the release of an organisation's financial statements but I do not think it is the Data Protection Act. A private company isn't obliged to publish it's affairs openly but I believe that they must file a copy officially which are accessible via
Companies House for a charge BUT it is not uncommon to find that such information is filed very late.
It would seem that if a union is run in the interest of member's interest access to information about the running of the organisation should be fairly easy to access.