The second property had no link with the first when it was built.
It was some years later that we knocked through to the stairwell of the property 2. to give extra space in shop 1.
Then dad died, brother had first propert (1)solely, I had the new propert (2) which included the stairs solely having nothing to do with the ownership of propert (1).
I retired, keeping property (2).
Then brother sold property (1), brother never mentioned to the & buyer did not ask about the stair access, until I told him.
What if any are his rights to use them.
I have offered a short term lease to give him access, so I can sell it in the future with its own private entrance.
He fills he should have unrestricted access for ever.
Does the other flat have access other than through the shared stairwell? If not then you would be a real @arse not to grant a permanent row. A court might see it that way as well!! And if your neighbour were to open a dispute over this it would have to be declared on the hips (or whatever it is) and would make it more difficult to sell with the row.
Your comment 'we knocked through to the stairwell of the property 2' etc especialy if there is no other access gives the impression that access was granted! and that at the time both properties were under joint ownership by all brothers and father.
How long have the flats had the link? If no verbal agreement was made, depending on time scale, he may have a right to access now.
I cannot see a problem with it being a duel access for both flats! As said earlier shared entrances are not normaly a problem for flats unless you make it so.
Personaly I would expect unrestricted access as well.
What does your solicitor say?