It's hard to advise on a forum without the full facts as every adverse possession situation very much turns on the specific facts of the situation.
However, a couple of basic principles might help you.
First, the limitation for a claim in adverse possession is 12 years, so based on what you say, it would seem that time hasn't run. You used the property up to late 1999 (you were in receipt of rent), so only 11 years has run, on the face of it.
Exclusive use of a property is key to establishing adverse possession. There is a risk that if you don't use it, you will lose title, so using it would be highly recommended. It would then be up to the squatter to prove his title if he wants to remove you. Bear in mind thatv asking for permission from the squatter may be constred as being tantamount to acknowledging his title.
Do get legal advice from a competent solicitor with the full facts specific to your situation. Don't rely on an internet forum for advice relating to something so valuable.