[rikur_]

If you had an area of garden above the sewer / surface water drain pipes that had recently started to dip (I reckon it is now about 5cm lower than it used to be). Is it reasonable to assume a leaking pipe and get the pipes inspected? The pipes date from 1995, so I'd have expected any settlement to have long since happened, and therefore I'm assuming recent movement is probably leak related.

Is it just a case of picking a 'CCTV drains inspection' firm off Google, or are there specifics to watch out for?

[johnm]

Is the surface water and foul water in a combined system running into a main drain? If so it is just possible that a leak is washing soil through the pipework. If you've got rods I'd give those a try first.

Camera work can usually be done by a local franchise of Dyno rod or similar.

[ROG]

Have you had a look at your home insurance. I was with homeserve for drains etc when discovered that my home insurance covered drains which are on my property.
Worth a check.

[Paul_Sengupta]

https://www.ebay.co.uk/itm/383963044819

[Flying_john]

I dug a rainwater soakaway about 20 yrs ago. It was only last year that a small sink appeared in the lawn above it.

So at the moment it has a couple of inches of topsoil and some grass seed firmed into it and I will wait and see !

It can take years for soakaways to settle.

[rikur_]

Is the surface water and foul water in a combined system running into a main drain? If so it is just possible that a leak is washing soil through the pipework. If you've got rods I'd give those a try first.

They are separate surface water and foul water pipes that run parallel along our boundary to a pair of concrete chambers where they merge with our neighbours drains/sewers (and are adopted from that point)

https://www.ebay.co.uk/itm/383963044819

I'm not sure I'd know what is within / outside of tolerance though. We had a cracked drain at house #2 that was seeping into the underfloor void ... I saw the video inspection, but I would have struggled to identify the point of interest.

[Charles Hunt]

CCTV is a 'tech' way to go. If you're concerned you could buy some drainage rods from Wickes and see if you can get those through as a starting point.

[johnm]

If you have rods I'd start by rodding both having lifted manhole covers and looked for silt in those downstream of the sag.....