Newly purchased property; unoccupied insurance required?

I'm in the process of buying 3 properties. 2 are tenanted, 1 is vacant.

I'm aware that many insurance companies allow unoccupied periods within the normal scheme of things. These are limited, the one I had on a previous property was 45 consecutive days.

My question: as the property will be vacant from day 1, can I take out a 'normal' landlord's buildings cover; in the hope that I find a tenant within 45 days (quite likely in the current market). Or, do I need to take out an expensive vacant policy for a short term, then a standard policy when someone moves in?

Thanks.