If you agree that the person in the flat over the kebab shop is a relevant person then you must agree that everybody in purpose built private flats with a common escape route is also relevant. Do you then have to gain access to every flat in order to carry out the RA.
No you don't need to gain access to the flat. You just need to ensure that the occupants are protected from a fire in the premises you are assessing.
In purpose built flats you
could argue that since compartmentation is in place they are not relevant persons as a fire in the premises could not affect them. However, since their 'relevance' seems to depend on the existing protective measures then I would argue that they are relevant persons.
The kebab shop is very different to purpose built flats, which is why they have an issue in the first place. The very definition in the legislation means that if there is anyone in the flat above the shop and they could potentially be affected, then they are relevant persons. (Regardless of any difficulties that it may pose an assessor.)