Hi Toby,
At the previous house we lived in (which was half a mile up the Fairlie Moor Road from where we are now), I made the dreadful mistake of putting up a 35ft polytunnel on an area that didn't have the best drainage in the world. During the summer it was fine, except for the fact that when you watered, everything seemed to get really soggy. To the end of summer, when the rain started, all hell broke loose, and my beautiful polytunnel became a complete mess!!! We could hardly even get into it as the path to the door was so bad. However, this problem would've easily been solved by putting drainage around it, but by that time it was becoming apparent that our landlord was a little mentally unstable and we were probably going to have to move (the house was as wet inside as the polytunnel was and she wasn't doing much to sort it). If you need help with drainage I know a guy in the Dalry area that's really good.
Do you need to put it in your field, could you not put it in your garden? If its in your garden you don't need planning for it, but as far as I'm aware you do in a field, just as Hermit has said.
Planners were very friendly to chat to..and now all propositions/drawings sent (again informally at this stage).
I don't know if you've dealt with planning in NA before, but they are a complete nightmare!!!
Plough and put a thick covering of mature horse manure & cover
Is the area predominantly clay like my end of the town? If so, I would avoid horse manure for now as it won't be good for helping with waterlogging. If you want to do anything at the moment, I'd be more inclined to dig through leaf mould, or something that's likely to make the soil more friable. Again, I'm learning from our mistakes here, the area that our polytunnel was on had horse manure spread over it 6 months prior to us building the tunnel, and it just seemed to compound the problem. The pigs sounds like a good idea, but again, I think I would want the drainage sorted first.
Good Luck!
Denise