most of what woozer says is correct here - check the terms of your insurance for the unoccupancy clause - you should always advise your insurer in any instance when a property becomes unoccupied just to be safe - unoccupied properties pose a significant risk compared to occupied properties.
If the leakage was as a result of the property owners upstairs negligence then you will be able to claim from their PL extension on their home/let property insurance - presuming of course they have this insured - however, if there was no negligence on their behalf - ie they had maintained the property correctly and were not at fault, then this is just something your insurers or you will have to take i am afraid - if there is no negligence there is no liability and no PL policy will pay out