If the case is that the mains flow - measured at a full flow tap immediately at the point the your blue MDPE ,that's connected to the district main, is flowing 10L/Min then the restriction would have to be downstream of that. The only thing downstream is the district mains.
The thing I can't get my head around is that the district mains shouldn't be that small a pipe nor restricted and as such, at that dynamic pressure, it should definitely be flowing more than 10L/min. To only flow 10L/Min the dynamic pressure in the mains should be a lot lower than you are seeing or the mains flow is restricted somewhere. Have you checked that the outside mains stop tap is fully open?
