Trade Union Collective Bargaining Rights
A recognised independent trade union has the legal right to obtain information (including pay and benefits, conditions of service, manpower statistics and productivity and financial data) from the employer if the trade union reasonably needs such information to conduct collective bargaining negotiations.
There are exceptions for any information which you, as the employer, can reasonably claim to be particularly commercially or legally sensitive, or if obtaining such information would be disproportionately difficult.
There are Central Arbitration Committee (CAC) and Advisory Conciliation and Arbitration Service (ACAS) procedures for settling any disputes as to the information which you are obliged to disclose.