Hi GL,
I assume as its they are flats you share the same Building Insurance which the Freeholder arranges. The policy will normally cover Loss of Rental Income if there is a damage claim eg a fire. Under this section the downstairs landlord could possible claim from the policy for the period the downstairs flat was uninhabitable and during this period the tenant moved out/into alternative accomadation.
For the downstairs landlord to sue you personally for the loss of the tenant he / she would have to prove you were legally liable. To do this they would have to prove you were legally negligent, this in effect means they have to prove you could have "Reasonably avoided this". Just because there was a leak does not necccessarily mean you are legally liable so there is no guarantee he will win the case.
The building policy you have probably includes free legal advice on it, there is normally a telephone number for this in the policy next to where the phone number to make a normal claim is. Give them a ring and get advice from them.