Leak on party wall

lrp79

There is a localised leak in my basement on a party wall in the middle of the property. I have had a trace and access company visit, but they were unable to locate a leak in my property. It has recently got worse, and after 3 or so weeks of completely dry weather, there was more water down there than seen previously, suggesting to me that it is unlikely to be groundwater?  The agent (the property next door is let) believes there is nothing wrong, with no sign of an issue on their side. 

They have some interesting plumbing on the party wall, also their property is properly tanked with a sump and pump. My property has eggshell type plastic behind the plasterboard, but it stops an inch or 2 above ground level and there is no sump/pump system. I wonder if there was a leak from their side behind their tanking, if that could be why /I'm seeing water ingress but they are not. I'm keen to establish if it is a leak from their property before embarking on the expense of fitting a sump and pump here. 

Is a proper investigation something that can be requested under the party wall act? I have offered to pay for a plumber to visit, but they say it's a waste of time/money.

If I do need to install a sump and pump, will I need a party wall agreement?