HOWEVER, if your dates are not accurate, and the 12 years ended past 2000, then the Human Rights Act 1998, which came into force on the 2nd of October 2000, will play a major role due to the first protocol in that act. Furthermore, if the 12 years ended past the 13th of October, 2003, then the Land Registration Act 2002 will basically estopp you from claiming the land as your own. It seems to be the case, that it is nearly impossible to gain the land, as the 2002 act abolished the limitation period, and you have to apply to be registered as the proprietor owner. Furthermore, the paper title owner will be notified, and will have 65 working days to respond to your application. If he does not respond, the property is yours, if he does reject, then you will not have the property unless you can satisfy one of three conditions a) estoppel b) the squatter is legally entitled to the land c)there is a boundry dispute.
However, I am concerned as to how the other person was able to register the land without showing good title! Especially as you hold the deeds to the property! Are you sure that other person registered the legal estate, and not an equitable interest? As unregistered land requires most of equitable interests to be registered under the Land Charges Act 1925 in order to be bound upon a bona fide purchaser for value!
So in summary, in all likelyhood, this is fairly easy to resolve. Get a decent solicitor who can write a letter to the registered owner and the registry office in plymouth to make you proprietor owner of the estate.
If your wandering about my credit worthiness, I am a law student who has just finished writing an essay on adverse possession, so i know what i'm talking about.
Hope it all runs smoothly, and you regain full control of your property!