There is a very large element of creative accounting possible in inter-company charges. For example if the main company send consultants to work on behalf of the airport, they can charge them pretty much what they want
A debt to the airport is an asset to Balfout Beatty plc which helps maintain the share price at a time when things are difficult for them.
My question would be why the council, as minority shareholders, were not kept in the picture regarding this debt to the main company.