Owner of flat upstairs refuses to fix leak

I live on a ground floor flat and have had my bathroom damaged several times by leaks from the flats above. The last case has been dragging on for the past three years.  There is a leak coming from the flat directly above me.  It caused great big patches of mould on the external wall of my bathroom and damage to my bathroom ceiling.  It also makes my bathroom suffer from damp and mildew.  Worst of all is the noise from the leak.  I believe the leak is dripping down from waste pipe onto my steel bath and this creates such a noise that I woken up and can hear it even with the bathroom door closed (which I am loathe to because of the damp).

I complained for the first time over two years ago.  The flat has been let out to two or three different tenants over this period.  It is owned by a man who lives in Nigeria.  His friend who lives in East London looks after it for him.  I called the management company who wrote to the friend.  Nothing happened so I got the council involved although this is not a council owned flat.  This seemed to spur the friend onto telling the tenants not to shower which helped but hardly fixed the problem.  I have called the management company several more times; they have written to the friend some, saying they can't do anything more than that.  The friend refuses to accept that there is a leak - as the leak is going down the waste pipe he will need to dismantle part of the bathroom to access it which he appears unwilling to do.  The tenant tried to tell me that the mould patches on the wall were dirt (!), and that he was a qualified builder and had taken off the bath panel and there was no leak.

The noise from the leak is worse than ever.  The tenants above rise at around five and shower and so I am woken up by the incessant thumping noise from the water dripping.  I am ill from stress.  What can I do to make somebody fix the leak?  Surely this amounts to criminal damage?  What are my options? I just want some peace and to be able to redecorate my bathroom knowing it won't be runined again in a matter of weeks.