Further to my question, after a quick google, it appears that renting to a solar developer gets you about £800-£1000 per acre per year.
5 acres is enough for 1MW of panels (at a cost of probably half a million quid), so assuming you got on average 3 hours a day full power (big assumption) and you can get 5p a unit, about £55k a year in generated electricity. So 10 years to get to break even.
If I am reading the charts right, Dorset gets 3kwh per kw of panels a day, on average.
I have no idea what grants are available or what tariffs bigger generators get.