No Water after Mains Switch-off, What to Check?

Water co recently turned off the local supply to carry out some work - unfortunately the supply did not come back on fully, meaning just a trickle/drip from the cold taps - all neighbours appear fine. The suggestion is that sediment or silt has got into the pipes and blocked a tap/valve somewhere, but after three plumber visits (contractors to the water co) we have no idea where - the water main outside supplies three different properties, and there is a stopcock on each split supply as it enters the building - “ours” has been replaced with a lever valve with no effect. Stopcock to the combi boiler has been replaced as it was seized, again, to no effect. The plumbers made various attempts at blowing back down the pipes with a compressor, and (three days later) we did get more flow from this, although still not enough to turn on the combi boiler for hot water. There is a brief rush of rush of pressure when opening a cold tap before the flow goes back to a trickle.

The issue is that there is no evidence of a stopcock or valve anywhere else (eg under the kitchen sink, bathroom basin, or where the washing machine plumbing is) - the cold mains water appears to enter the (1st, 2nd and 3rd floor flat) via a 1st floor bathroom from boxing (with no access hatch) on a communal stair, and there appear to be two copper pipes disappearing underneath the tile-over-ply bathroom floor which are visible from the bath and lavatory plumbing. The bathroom basin taps are supplied from PVC pipes which also disappear under the floor.

Nine days later, the plumbers are intending to rip up the bathroom floor with the expectation of finding a stopcock or valve, which seems pretty unlikely to me - is it not more likely to be found inside the boxing before entering the flat? Could there be a blockage from sediment which isn’t at a valve? Suggestions for a course of action welcome…

What work was done?
Soft cooper pipe 15mm in ground? That's what runs to our 1960s property.
Is it possible the work caused this 15mm pipe to get pulled and crushed?

Not sure exactly why the water was turned off, but it was something to do with the mains being replaced in the adjacent street - nothing to do with this particular property or block. There is definitely good pressure and flow to the building from the mains, as the neighbours have no problems, supply from the mains is 22mm blue plastic piping, which appears to change to copper at some point before entry into the flat (from previously looking behind boxing).

Could there be a solenoid valve hidden somewhere?
A type that detects water pressure and switches off when pressure drops as it thinks there is a flood?
Trouble is, finding it.

Has the pressure been checked at the lever valve?

Looking at the rest of the plumbing, I don’t think there’ll be a pressure valve.

The new lever valve is directly after the mains feed splits, so I’d expect it to be OK given the other flats are.

RealPolitik said:

The new lever valve is directly after the mains feed splits, so I’d expect it to be OK given the other flats are.

Click to expand...

So not tested and maybe reluctant to?

Don’t follow - the old stopcock directly after the split from the mains was removed and replaced with a new lever valve, there is definitely flow and pressure immediately downstream of that. Why would that be a problem to test (rather than going to all the effort of pulling up floors)?

So the lever valve is for all flats, not just yours?

No, the lever valve replaced an ancient stopcock (one of three) immediately after the split from the single supply line for the mains, it was the first thing the plumbers did. The other two flats still have the old stopcocks with no issues.

So how do you know that the new lever valve isn’t where the fault lies?

Because changing to it made no difference at all - the only thing that did was using the compressor down the cold feed to the boiler (I think). I’m really hoping for a way to check for blockages in accessible sections before pulling up floors, but I’m not sure if that’s feasible (eg from boiler feed to just outside the flat, then from there to the first section on the ground floor, then to the initial pipe off the mains).

Do you get good flow if you disconnect the pipe immediately downstream of the lever valve, at least this will tell you if the blockage is downstream or upstream of it?.

Is your flat on the top floor? Are your neighbours' on floors below?

RealPolitik said:

and (three days later) we did get more flow from this, although still not enough to turn on the combi boiler for hot water. There is a brief rush of rush of pressure when opening a cold tap before the flow goes back to a trickle.

Click to expand...

The brief rush indicates that sufficient pressure is available, but that it's flow has been restricted.....a partial blockage.
You said several water board plumbers had attended, and blew air down the pipe. Was this compressed air flowing back towards the 'branch', and was the pipe at the branch open to atmosphere?

Johntheo5 said:

Do you get good flow if you disconnect the pipe immediately downstream of the lever valve, at least this will tell you if the blockage is downstream or upstream of it?.

Click to expand...

I don’t think that’s been tried (I had suggested it), because they weren’t clear which 22mm blue pipe belonged to which property in the void before they disappear behind the wall.