Although, it's my understanding that the contractual agreement that West Sussex/Mid Sussex District Councils hold with Gatwick regarding no development of a second runway there is with BAA - and would therefore be null and void in the event of a sale,
Absolutely correct on the first part. The 1979 agreement is between the airport owners and the council. Not sure about whether or not that agreement has to transfer as part of a sale, but the government can overturn and build any time they like. They make the rules.