I'm a leaseholder of a flat. I have received a demand for a share of the buildings insurance premium

You should refer to the terms of your lease in order to establish who is responsible for insuring the building, what the cover should include and how the cost can be recovered. Usually the lease provides for the landlord to arrange the insurance of the building (not the contents) and charge the cost as a service charge. The cost of the insurance may be challenged before or verified by the First-tier Tribunal (Property Chamber) as with other service charges.