You are allowed to site a caravan on your land for tea breaks, to get out of the rain, change clothes, eat lunch etc etc all under Health and Safety at work. It refers to employees but you need those facilities too.
http://www.hse.gov.uk/simple-health-safety/workplace.htmThe caravan is incidental to the use for the land and you DO NOT need planning permission. Where else are you to keep a first aid kit and animal medicines/equipment?
Furthermore you are allowed to sleep in the caravan for up to 28 nights for the sheer hell of it AND you are allowed to use it as a stockman's hut and again sleep in it at lambing etc etc. It may also be used by seasonal agricultural and forestry workers. The caravan can remain on the land all the time therefore it can be a static or a tourer, neither are permanent fixtures even if a tourer can be moved more easily, a static is still a mobile home.
It's not the caravan being there that changes the use of land it's what the caravan is used for and quoting the above will cover you if you're ever challenged. Farmers do get quite a bit of back-up when it comes to animal welfare etc.
Be careful of building things like decking around or laying hardstanding/patio, however practical or attractive, they can cause issues such as making it look like a residence and their degree of permanence.
Unfortunately most councils' POs don't know most of this OR think that you don't know, so by default they tell you to apply for PP and pay them a hefty sum of money!!! You don't even need to tell them you're doing it, so I wouldn't bother with them again.
As you can probably tell I am not a fan of planners!