The reality of-course is that if you are 5 miles from the nearest human habitation - do what you like, nobody is going to know or care. If on the other hand you live close to some expensive houses, expect complaints from about the second take-off. Legally however, yes you have got the 28 day rule - and with care you can manage to gently exceed that by use of careful noise and overflight procedures, not allowing anybody else in, until eventually you can prove umpty years of use without objection and apply for formal change of land useage.
G there is a Hamlet of houses about 3/4 mile away with a church alongside the field. He could always paint it red with a cross on the side and pretend its some local air ambulance service for residents failing that a visit from the heavy mob
Pace