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

[RealPolitik]

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

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?

This flat is 1st floor and above - as far as I’m aware, the feed from the mains also supplies the ground floor commercial premises and two separate flats above a separate commercial property next door (shared flat entrance door and stairs, where the pipes are briefly visible, developed at the same time).

The compressed air was flowing back down towards the branch, but I don’t think the pipe wasn’t open at the branch/mains connection.

 

[MeldrewsMate]

The compressed air was flowing back down towards the branch, but I don’t think the pipe wasn’t open at the branch/mains connection.

If the pipe wasn't disconnected at the branch end then the compressed air isn't going to blow anything out because it wouldn't flow.

 

[RealPolitik]

My thoughts (but something, somewhere, moved because a little bit more water started coming out of the taps).

 

[CountryFan]

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?.

Kind of what I was asking

 

[Johntheo5]

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.

You know where your new lever valve is, maybe disconnect here and try using compressed air to push any debris back through the system, they could connect to different outlets with the others shut off.

I suppose there is no way that a combined lever/check valve was installed, and "backwards"??.

 

[RealPolitik]

Problem solved (ten days later) - there was an old stop tap buried underneath the bathroom floor, which wasn’t fully open - plumber got it loose and the water’s back. Quite a bit of metallic-looking dark sediment came out of the taps, and the cistern fill hose was blocked with it.