I shall be as interested as you [member=212774]frogstercouk[/member] to hear advice** from TAS members (welcome to TAS by the way and good luck with your project/s).
Rented/leased land can count towards the 5ha threshold, but, like you, I have never found an answer to the question** of how long does one need to rent/lease an amount of land - in other words what will your planning department be looking for to establish a serious rental/lease of land (not just a sham arrangement to by-pass full planning permission)? I did actually flag this issue/question here once before, but am none the wiser to this day.
As to permitted development on 5ha+ : I'm sure you are aware that PD still requires notification to Planning. I've no idea what percentage of PDs go through without being contested, BUT they can be contested if Planning considers the proposed dev' is not in keeping with it's location etc etc etc and end up requiring a full planning app'. And I seem to think, but am not sure, that land without any structures already will be reviewed very carefully by Planning if they receive a PD notification.
Something you might ask yourself (in deciding which way to go) is whether your proposals will be obtrusive on "virgin land" close to a village or whether they will be hardly noticed/noticeable due to topography, with good access and not bothering anybody etc etc etc.
Where are you by the way ? (It helps to know as regards what advice might be given based on potentially local experience.)