Hi [member=24875]SteveB[/member]. Not sure how I missed your post: here are some belated comments.
Firstly, I'm wondering how your land search is going (?): finding that perfect bit of land in Cornwall to meet your aspirations is not easy! Such parcels do not come up often and, when they do, you will need to be v v quick and expect to pay a good price: not as much per acre as a couple of acres for a pony paddock, but ...More likely than not, those 5 ha or so that you want will have been split into smaller parcels to go on the market.
28 day rule: usefulness might depend on whether your land is adjacent attentive/disgruntled neighbours!
Re Cornwall Council planning: I don't think they are much different to any other Local Authority. That said, from my limited experience, they do take support for new rural businesses seriously. On the other side of the coin though, Cornwall is a popular place to be (unfortunately becoming over-populated perhaps) and CC have no reason, I reckon, to encourage/accept development proposals that push the envelope. I would say/guess that any future development proposal/requirement you might be trhinking about for "living on the land" will be v v carefully scrutinised and, probably, refused in most circumstances.