Trespass by Housing Association

Hi, hope someone can advise on this. I am a leaseholder in a block of six flats owning a top floor one. With this flat comes the front garden.

Back in the middle of the year the housing association fitted a new condensing boiler to the flat below us (ground floor) and installed the pipe which carries the condense waste (acidic) into our front garden. I have made the housing association aware on several occasions that they have trespassed and have no authority to put this pipe discharging into our front garden flower bed.

I spoke with their local neighbourhood housing officer two weeks ago who assured me that she would have this rectified and moved.

Last week I sent her an email stating that if this pipe was not removed by Thursday 26th October that I would take further action and escalate the complaint.

My question is that how do I stand legally on this and what further steps can I take to get the pipe removed?

I know that I shouldn’t remove the offending pipe myself as this is part of the workings of the boiler.

Thanks
Barry