I hope I've got a link to a site that will help you,
AIRPORT COORDINATION LTD
Airport Coordination Limited (ACL) is an independent organisation which undertakes the allocation of aircraft take-off and landing slots on behalf of UK airports, in accordance with UK and European Commission regulations and IATA scheduling procedures. ACL is the world’s first independent agency responsible for slot allocation – in most countries this work is still undertaken by the national carrier or largest airline at the airport – and other European countries , including France and Denmark, have used ACL as a model for establishing their own similar organisations. ACL is an administrative body and does not involve itself in matters of policy, although it is called upon from time to time to submit its views on policy issues and to participate in consultation procedures
ACL is an independent Company owned by 11 of the UK's leading airlines who each own one share in the Company and make a contribution towards its operating costs. ACL is a non-profit making organisation.
Current owners are KLM uk, British Airways, British Midland, Britannia, Virgin Atlantic, Monarch, Air 2000, Airtours International, CityFlyer Express, JMC, GB Airways.
The site should be of use, it even lists "spare" slots at Standsted and Heathrow (not Gatwick, sorry)