I am not a solicitor, but ... you have had 23 years of uninterrupted unchallenged occupation. Ask your solicitor about the merits of pursuing a case for Adverse Possession (aka Squatters' Rights) under Section 49 of the Registration of Title Act 1964. This would seem to be the most direct way to get a saleable title, in the absense of any clarity on what exactly happened title a quarter of a century ago. In my view the original solicitor who acted for you and the IP should jointly cover the costs of any such action.
See also No progress without deeds and the mystery of the lost deeds .
See also No progress without deeds and the mystery of the lost deeds .