An 'adopted' road means the local authority have accepted it as part of the public highway and they are responsible for maintaining it.
Conversely, the land forming an unadopted road is owned by someone else. It could be the freehold landlord if these maisonettes are leasehold, or it could be that all the properties on the development own a share of this land. It seems unlikely to be that one person 'owns' the space outside his maisonette, but if he did you would see it if you downloaded the title register and title plan for his property using the link Buenchico gave you.
Easements are a different thing. An easement giving a right of way over another's land entitles one to pass/repass over it. It does NOT give a right to park on it - only pass over it. If the road is owned by the landlord or another party, as The Builder says, it is 100% certain that you have an easement recording that you can pass along the access road. You tedious neighbour will have the same. But it is virtually unheard of that he gets the sole right to park on this road - only in a space allocated when the development was completed - not where it thinks he can just get his paintbrush out to create.