Leak in Leasehold flat under concrete - who is responsible?

bowden_lucy

Our flat has a leak that has damaged our bathroom floor and is affecting the flat below.  The managing agent told us we needed to investigate as it was our problem.
We got a plumber who stripped up the floor and concluded that there was no leak from any visible pipes/drains in our bathroom and given the location of the water, it was probably due to the waste pipe leaking up from below our flat. He checked the boiler and noticed that the pressure was low but concluded that due to the amount of water on the bathroom floor, the leak could not be caused by the heating system.  
We passed it back to the managing agent who got a plumber out that agreed with our plumber with regards to to waste pipe as probable cause and also that the a leak in the heating system would not cause that amount of water.  However, the managing agent  decided his report was not conclusive so then got another plumber out. Their second plumber said it could be the waste pipe but having checked the boiler pressure which was low, he concluded it was due to the heating system. we explained that we knew there was a boiler issue causing the pressure to drop but this was another fault and that a separate plumber had been engaged for that.  However the managing agent said we needed to get the heating system fixed to prove it was not the cause of the leak and then to continue with the investigation.
The heating system is fixed, we engaged another plumber to investigate who has now said that due to the amount of water and as it has continued to leak, it is probably the hot water or mains pipe that sit under the concrete.
Although these pipes serve only our flat, they sit within the structure of the building ie the concrete.  Should we be engaging a plumber to dig up the concrete floor or should this be the managing agent? (two plumbers have told us that they would not be willing to dig into the structure under our instruction as we are only leaseholders)
Also, who would be responsible for the cost (or the £2500 excess as it should be going through building insurance)?
The bathroom has been out of use for five weeks with the back and forth with no leak yet identified.