In all likelihood you are primarily responsible as the owner of the flat where the leak originates. If the source is the result of poor workmanship and the installer can still be reached then you might be able to get him to accept at least some liability and to that extent it is advisable to have him revisit the premises to investigate the leak. In any case it is best for you to notify your insurer - there is likely to be an excess on any claim, i.e. an amount you will have to pay and the insurer pays the rest (and charges you a higher premium next time).