Horses - they don't make any planning issues any easier !!! Hopefully other TAS horsey folk will be able to offer some thoughts, but it does sound like you might be in a bit of a bind.
You haven't [member=4534]caz[/member] still actually said what you propose for a permanent trackway surface. (Is that where the problem lies ?)
However, you will have to do something, obviously, and I can only go back to the suggestion of just getting on with it.
THAT SAID, don't make the track-way too "permanent" such that it would be a right pain (costly) to remove (or plough-in

).
Not sure what you could use, but a thorough mulching of the "track" with bark chippings or similar (not exactly cheap) might get you through in the short term.
OF COURSE, the question to be asked for the long-term (it has occurred to me) is whether your declared use of the land/barn still counts as agriculture. As you will almost certainly know: only the simple grazing of horses is permitted on agri' land. Re the barn, it might depend on what you have declared with regards to use of the barn (e.g. storage or stabling) !
Subject to any further/better TAS advice/suggestions, I would now suggest having a good talk with your planning officer (if they will pick up the phone these days) and explain, more fully, your appreciation of the situation -
especially IF your declared use of your land/barn is still classified as agriculture.
If your land/barn use has to be reclassified (due to intended usage falling outside of agricultural use) I would suggest renting a field short-term for the horses and re-establish your land's agri' status by ploughing it up and then re-seeding and creating (at the same time) a "farm" track to the barn !!?
I believe I've exhausted my thoughts on your problems, but obviously will be interested in how it all pans out. Very good luck.