Download a cheap or free voice/noise activated recording app for your phone or computer and send the people upstairs a note telling them that you will be having guests at such and such a time and you would appreciate them keeping the noise down. When they don’t it will be recorded and time-stamped. That should be all the evidence you need for a tribunal. There are a few apps around that will do this for you – one of them is called Snooper but I can’t get on to its website.