Flat Insurance that has no management compnay

Other

Hi - I have a groundfloor flat in a block of 4 flats (2 groundfloor & 2 above). I own the freehold for the flat above and they own the freehold for mine. There is no management company involved. This arrangement seems to be widespread in the area. As far as I am aware everyone organizes their own buildings insurances separately. The insurance companies that I have spoken to do not seem familiar with this type of arrangement, some of them think we need to arrange insurance for the whole block ie all 4 flats whilst others think to insure our own is sufficient. It was suggested to us that if we only insure our own and our tenant burnt down the whole block and one or more of the other flats had inadequate or non existent insurance we may be liable.
Have you come across this before or can you offer any advice ?