I found this in the Daily Mail online.
'Normally, a squatter must possess land for 12 years before claiming ownership under adverse possession.
Making a claim has been made more difficult since 2002 - squatters have to inform the landowner of their intention to claim possession. But if you had already been squatting for the qualifying period before 2002 - if you enclosed the land before 1990 and have occupied it ever since - this will not be necessary.'
Read more: http://www.dailymail.co.uk/property/article-1348664/HOME-TRUTHS-How-long-I-claim-land-own.html#ixzz2M8ZDn064
We had a house with an extra piece of enclosed land once. We were told that after 12 years, no-one except the owner could claim it from us and that, after 20 years, ecen the erstwhile rightful owner could not reclaim it.