We live in a leashold pupose built first floor flat. Together with the other leasholder (who occupies the ground floor) we have purchased the freehold. The freeholder had previously provided the building insurance with us both paying half of the cost. The current insuance provider will not swap the policy into our names and so we are having to find a new policy. So my question is: Is it better to get a joint policy to cover the whole of the building (which is part of a terrace of purpose built flats), or get each get separate building insurance?

It depends on what quotes you can get. You might have to search around to find an insurer who will provide cover. It will help the insurer if the freeholder has an identity (like a limited company with 2 shareholders).