Insurance for flats

we own a number of flats which the freeholder insures in a block policy. can anyone recommend an insurance policy we can take out to cover issues that might arise whilst the property is let. In otherwords the tenants bathroom has flooded the basement flat. Both flats are let out, but who will be responsible to pay for the damage. Will it be covered by block coverage or should be also as owners have our own insurance, and in which case can anyone recommend where to go to get cover