This is only my opinion, but I see no reason why you should have to ask anyone's permission to "repair" your own barn on your own land. You say it is hidden in a wooded grove, so I'm assuming it's not in the public view anyway.
When you say it's collapsed, is there any part of it still standing? If so take some photos of where it looks its best and at its most usable and keep these, just in case the council come snooping round, to prove that you are actually repairing and not rebuilding.
Having said that, I would just get on with it. But whatever you do - do not ask the planners their opinion! You must have read on here the numerous accounts of the hoops you may have to jump through just so they can justify their jobs.
And also, do not remove the ruins and then apply for planning as by taking a building away you have then deleted any rights you might have to put up a replacement one on the site, even if the old structure is clearly shown on your deeds.
As I said - I would just get on with it!