Confusing mains water leak

[Ed21]

I've got two drains, one with branches from the house and the other for inspection.
The inspection one has clean mains water running through, while the house one is dry,
but can hear the same running water (supply at house turned off). 

Clealy have a leak, but this is plastic drain pipe and any water supply is also plastic.
It's never been distiurbed or drilled into etc. and no trees or roots anywhere near. 

Can't figure out how mains can get in the soil pipe. 
Anybody got any ideas?

Cheers Ed 

Edited 1 hour ago by Ed21

[ProDave]

Pictures and diagrams?

 

Are you SURE the one with running water serves your property and is indeed foul drainage not rainwater?

 

Do you see more water flow through when you flush a toilet for instance?

[Mr Punter]

So, house > IC1 (picking up house, dry) > IC2 (inspection only, clear flowing water) > off somewhere else?

 

Is the water meter showing any flow?

 

When you flush a WC do you see the flush in both ICs?

[-rick-]

10 minutes ago, Mr Punter said:

Is the water meter showing any flow?

 

Check this asap. Expensive problem if left. If meter is spinning, turn off at meter and see if leak stops. (If you are not on meter isolate at external stop-cock anyway to check flow)

 

If meter not spinning, then likely coming from another house. Report as leak to water provider and let them trace it?

[saveasteading]

8 minutes ago, ProDave said:

Pictures and diagrams?

A plan showing the house, all manhole and grille covers, & directions of flow.

 

As well as flushing as a test, a piece of toilet paper proves where it is from.

 

There is a product called drain trace. It is brown power that shows as luminous yellow when diluted. With that you can trace from toilet pan through each pontoon it's journey.

 

It's unlikely, as you say, that a mains leak is getting in there, but perhaps an overflowing tank or wc.

[Ed21]

Thanks fopr all the replies  

When the mains water supply is turned off in the road the flow stops, so this is the souce. 

Water is tuned off at the house and toilet flushed, so no supply from there.
House drain is dry. 

It's a single soil pipe connecting two drains and no way mains water can get in there unless 
both soil and supply has broken at the same place, which sounds pretty weird. 

[Russell griffiths]

Get a bloke out with a drain camera. 

[Ed21]

Yeah that's the next step. Pipe is about 1.2m down, so gonna be a lot of fun once leak is found.   

[-rick-]

10 minutes ago, Ed21 said:

no way mains water can get in there unless 
both soil and supply has broken at the same place, which sounds pretty weird. 

 

If both pipes are close to each other and there's been ground movement it would make sense for both to be damaged. Hopefully this narrows down the search area to only those where the two pipes are close to each other?