As Postdog says, if you and your neighbours plots are registered land for a couple of pounds you can download the title plan of both your plot and his plot. That will show you with whom the land in question is registered. It is just possible that the parcel of land has a separate title number, but that is unlikely.
If you also download the text of the title title itself it will say if there is an easement (a legal right of way - in this place on foot for him to go to the piece of land. No easement, then he can't access it though your land.
Regarding the wall, it depends what is on the other side of this wall. If it is public highway, bridleway or footpath, he could knock a hole though it and gain legal access that way. If you or another landowner own the land on the other side then his plot is indeed landlocked and he cannot get to it. You have no rights on it however until you have maintained it for over 10 years when you could start the process of registering it in your name. He cannot get to it (so in theory cannot erect a fence around it). The best solution for him would then be to sell it - but he may not see it that way.
You will almost certainly have to employ a solicitor to sort it out but you could do some of the groundwork to establish the facts yourself cheaply.