Section 187B of the Town & Country Planning Act 1990 gives local planning authorities the power to apply to the courts for any 'apprehended' (i.e. anticipated) or actual breach of planning control to be restrained by injunction.
Injunctions are the only instrument available for breaches that are apprehended but have not yet occurred.
The problems with injunctions are that their granting is at the court's discretion, and that a failure to get one is likely to lead to a heavy bill for costs.
It is often helpful that Section 187B (3) allows for injunctions to be issued against a person whose identity is unknown. The former Office of the Deputy Prime Minister gave advice to local authorities, in a letter dated 26 July 2005 concerning roadside advertisements, on the issue of an unauthorised use moving from one field to another.
The letter mentions that the 'site' in question to be enforced against could be all land held by the landowner, and not just the field where the breach takes place. This could be a good response to the problem of people moving between fields. In addition, when applying for the injunction, the local planning authority should draw the courts' attention to persistent offenders and previous convictions, which would strengthen its case to get one served.