Indeed the OC has to meet the NCC/BCC requirements on common property stairway handrails that applied when the stairs were constructed.
And it’s probably not a bad idea to apply the current requirements (if they have changed) in the interests of general safety.
My OC has now installed 2 of them. I found it curious that handrails weren’t already installed but on checking the current NCC/BCC requirements, they aren’t actually compulsory for stairs that have a wall on each side of them – which was the case for us. We installed them anyway.