Problem solved. I suspected there was a sprinkler/riser that I couldn't find. After capping off all risers that I could find it revealed itself, broke ...

Thanks guys I did do a reverse flow as part of clearing the pipe but that was with an open end. I'll take your suggestions and work at it. Gonna take ...

I have a remnant of a reticulation system, 20mm pvc about 15m with 6 risers. Is it possible to connect a garden hose from the tap and have it work?