As I understand it the property is divided into two flats. One is occupied by Mr & Mrs Noddynoo, who have bought a lease. The other is occupied by somebody else. The freeholder (“she”) is not that somebody else. She lives abroad and has never lived in the other flat but she has told her insurers that she does.
Mr & Mrs Noddynoo contribute two thirds of the buildings insurance premium (which has three joint policyholders, the Noddynoos and “she” who lives abroad). Hope I’m following this OK.
In my view a false declaration has been made to obtain the buildings cover. “She”, as one of the joint policyholders, has declared that she lives in the property (in the other flat) and she does not. She is clearly an absent landlord who has sold a lease to Mr & Mrs Noddynoo for one flat and is renting out the other.
Noddynoo, you need to contact the insurers and acquaint them of the situation. It may be they are happy to continue with cover under the new circumstances or it may be they are not. One thing is fairly sure: if they are not informed and the building burns down leaving it a total loss you may find you will have no compensation for the lease you hold on a pile of rubble. Insurers very often make no extra charges or impose no new conditions when you inform them of changes (though I’m not sure that will happen here). They do, however, stick to the letter of the policy and the statements made in the proposal if faced with a payout of many tens or hundreds of thousands of pounds.
As a leaseholder you should have no need to be involved with the buildings insurance. You may come to an arrangement with the freeholder regarding payment but the property is not yours to insure.