There is currently 6.7 GW of gas-fired capacity and 0.9 GW of gasified coal capacity with planning consent. Time scales for construction are a matter for the companies concerned. However, the Department publishes an overview of generation capacity development in its annual Energy Markets Outlook Report, which is available at:
http://www.berr.gov.uk/energy/energymarketsoutlook/page41839.html
Copies of the report are also available in the Libraries of the House.