to 'liveandletlive' - (you wish!!)
1). The siting of a caravan for a purpose incidental to the agricultural use of the land (as a rest room, place of shelter as such) does not require planning permission (unless there is an enforcement notice in effect or an Article 4 direction preventing that specific use - which is unlikely).
2). Residential use of a caravan on agricultural land does require planning permission unless it is exempt under the Caravan Sites and Control of Development Act 1960, Schedule 1 paragraphs 2 - 10.
3). The garaging and restoration/repair etc etc of non-agricultural vehicles constitutes a 'Change of Use' and would require planning permission - but may now be lawful if you have been doing it for 10 years (and can prove it).
The Council should have copies of all enforcement notices on the public register (or in their archive).
Unfortunately the enforcement officer is entitled to snoop around as much as he/she likes and can require access to all buildings on the site - if you deny access - which is an offence - they can (and probably will) go and get a warrant - and then you'll sat on the naughty step - big time.