reading the 28 day notice stuff, I think it's just the land that's classed as agricultural. so I think it can include things like yards / hard standing - but not the house, nor the "amenity land". It does, however, also include non-adjacent rented land.
We're going through this soon too (well, maybe, seemy thread about estate agents...), but we read that book "field to farm" a few years ago and found it very useful for explaining the 28 day notice and PD stuff.
HTH
Adam