The number of members of trade unions is a matter for trade unions. The Cabinet Office publishes data relating to Civil Service facility time on a quarterly basis. The latest data is Quarter 1 - 4, 2014 and is available at https://www.gov.uk/government/publications/trade-union-facility-time.
The latest data available for facility time is Quarter 1 2015. The number of working hours taken by trade union representatives was 38,423.5. The cost of this facility time was £423,088. This represents 0.07% of pay bill for this period.