I'm sure someone must know on here so thought i'd ask!!

Basically i'm about to exchange contracts on my house first and have been asked to make sure that I have a cover note / buildings insurance in place for that date. Surely though the building insurance shouldnt start until the completion date should it otherwise there will be 2 insurance policies on that property at the same time before completion. (current vendors and myself).

Anyone know what the reason for me doing this is? What would happen if a claim needed to be made, who's insurance company would it come from being as i'm not actually living there / completed?

Cheers
Chris