I note your hopeful angle on seasonal workers [member=212617]JemimaJ[/member] . I am not an expert on permitted development for accommodation for seasonal workers. However, some thoughts ...
Firstly, I note you say "small plot" - small plots below 5ha have quite limited permitted dev' rights although any permitted below-5ha dev's (check for planning rules ont' web) do not need to be notified to Planning Auth' unlike all similar dev's for holdings above 5ha.
BUT then, please consider that any potential permitted dev' for seasonal workers accommodation also means only seasonally situated accommodation: as far as I know, a static caravan permanently left on the land for seasonal occupation does not count as "seasonal", i.e. temporary development.
Perris is almost certainly right in your case I reckon.