It is a tricky business insuring a flat. The biggest worry is that you might be insuring your flat being the sensible person that you are, but is Mr. Bloggs downstairs his flat too? A bit of an unknown situation arises if the worst happens and the whole places burns to the floor.
Your insurance company is covering you for the re-building of your flat and your flat only. If Mr. B hasn’t covered his place then what are the insurers going to do? They can’t rebuild your upstairs flat if there is no downstairs flat built underneath it.
You’ve also got the worry about that communal corridor inside with your front doors on either side. If something happens to that and your neighbour hasn’t got cover, then who is going to pay out for that. It is doubtful that your insurers would pay the whole lot and once again it is difficult to half do it.
The only solution to all these issues is to take a block insurance for your flat that will cover the whole property. This pretty much has no down sides. In general the policy is cheaper that taking out separate policies, there is no worry of someone in the block not being insured and any claims involving communal areas or across more than one flat is easily managed by one insurer rather than having to get two different insurers to sort it out between them or even worse and only one person has the cover in place.