Right, so your new explanation is different from the original - so that changes everything (to your disadvantage).
What you said originally was 'I have had this bit of land for 7 years now it has a static caravan on it'. What you actually mean is someone else has had this bit of land for over 7 years, that someone else has put a static caravan on it, but more recently he sold you the van.
When someone buys a house, they do not actually 'buy' the house, they acquire the plot of land on which the house is sitting. The fact there happens to be a dwelling on it is immaterial; one gets the dwelling as well for 'free' because it is permanently attached to the land. The value of the plot of land largely depends on the size of the house on it.
Your situation is different; you have bought a temporary structure on a piece of land where no-one actually knows who (if anyone) owns the land. Irrespective of what the bill of sale for the caravan says, you and the seller cannot just believe that you have acquired some rights to the land underneath. To transfer a land, a legal document called a deed must be transacted AND (these days) the deed must be registered with the land registry. All you've bought is a van.
What's worse for you is that the seven years that the previous occupier may have built up towards the minimum twelve years required to demonstrate adverse possession (always assuming the land is unregistered at LR - see my point 1) in the my first answer) are now probably lost and you are going to have to go through another twelve years of factual posession before you could lay any claim to permanent proprietorship of the land under it.
Meanwhile, a valid legal owner could turn up at any time and demand that you shift the van off his land.
As others have already pointed out, having planning consent to be able to live in it is entirely disconnected to the land ownership. You may well be able to live in the van without anyone noticing for over 4 years (after which time it is unlikely the planning authority could turf you out) BUT that still doesn't strengthen your right to ownership of the land. Indeed, you could well do that, acquire the residential right for a van on the land, but then find the real owner turns up and says' thanks very much, you've just increased the value of my land by establishing a planning right on it - now shift your van or I'll get a legal enforcement onto it'
My bet is that a local will inform the planning authority about the illegal occupation of the van and you will be shifted out of there inside three months.