This is the culprit!

Came home yesterday afternoon to a very strange sound in the house, as if we had water gushing from somewhere!  We dashed round checking taps & loos, turned water off at mains, checked the cistern, ran off some hot water - all with no success the sound continued!  Took up lid of inspection chamber - no water running through there - we were stumped!  B went next door to see if our neighbour could help, when he opened the door he said "Have you come about the noise!!"  Apparently some contractors had gone through the water main, the noise was the result of this in every house in the street.  Panic over! 
Took a shot of the innocent looking works today.