I spoke with Sir Norm last week at the NFFF. It seems that several fields, including Field 4, were sold "from under us" as he put it, two years ago but that they had contracts for last year. This means that they are not available this year, whilst the future is dependent upon a whole set of negotiations with the new owners.
Since F4 is a flood plain, at least they can't buld on it!
Interesting. That opens up a whole new explanation for the reduced capacity this year too. If they didn't know if they would be able to use those fields for camping, it makes a lot of sense to reduce the general festival numbers, and presumably there are fixed thresholds for planning which limited them to 5000. Thanks Andy.