In reply to Milesy:
Not Scottish, but prescription in English law would only be relevant if you were occupying the land without the owner getting involved - that means not charging you or asking to leave, etc. Assuming it's the same principle, since you've got a lease it won't apply.
Try contacting a lawyer, you might get (a little) free advice before they start charging.
Bad luck, hope it works out for you!
Post edited at 15:23