Heathfield's answer provides a good starting point but the qualifying period is not 7 years. For most unregistered land it's 12 years but for Crown land or some Church land, it's 30 years. (For registered land it's 10 years).
In order for the Land Registry to consider your claim to the title of the land, by adverse possession, four tests must be met:
* the squatter has factual possession of the land;
* the squatter has the necessary intention to possess the land;
* the squatter's possession is without the owner's consent; and
* all the above have been true of the squatter and any predecessors through whom the squatter claims for at least 12 years prior to the date of the application.
That last test might prove the most difficult one to get through. You need to show that you (or the previous occupier(s) of your house have been using the land (rather than simply having access to it) throughout that period.
If you think that you can get through those qualifying tests, I recommend the following:
(i) read this:
http://www.planningsanity.co.uk/forums/ap/ap.h tm
(ii) download Land Registry Practice Guide 5. (There's a link from that previous site)
(iii) if you still think you've got a valid claim, download Form FR1 (again, there's a link from that first site), complete and get a solicitor to witness the relevant declaration. Then submit it and cross your fingers!
Chris