Suspicious stain on paving slab

How deep is the drain pipe?

Really the only way to know for sure is to have a look, and the easiest way to do that is with a camera, either by getting someone to do a camera survey, or buying/hiring somthing yourself.

If it is just cracked, or has root ingress on a join, the drain can be lined with a resin liner, but I think that is out of the DIY scope.

It may be covered under your house insurance.

If the drains are running free, I would be inclided to leave it and see if it stays like this long term before digging anything up.

Well in the inspection chamber the pipes about 18-24" under the surface. So rising from that to meet the drain.

There's no trees near it - although from what I understand roots can travel quite a bit.

Hmmm hadn't thought of that.

As far as I can tell drain is running free. Pouring a bucket of water down it results in water out the other end !

Maybe, as an experiment, I'll swap slabs ... at least the Mrs will think I'm doing *something*.

That's a good idea, you dont happen to use a BBQ where this patch is do you, just thinking it could be fat that has soaked into the slab rather than water?

You would only get water on the surface if the drain was blocked. So first thing is to get the cover up and pour water down the drain and see if you can see it passing through the manhole.

If so leave it be for a while at least. The water may be a temporary spring or from a mains water pipe.

There's not a dripping gutter above the damp patch is there?

Or a net curtain visible where the MILF next door strips off?