Looking for some guidance on a manhole/drain - Getting confusing advice

[oggy_goodwin]

Hi to all, I hope someone can offer some sage and sensible advice.

In our back garden, where we are planning an extension to the house as well as repaving the garden, we have a manhole.

We've lifted the lid on it a few times, and
1. It doesn't smell like a sewer
2. There's usually just a trickle of water at the bottom of it and
3 - at the bottom - it has a base that looks just like this one in the picture that helpfully corresponds on the main website (thanks - and we haven't figured out the photo upload).


4. The man hole is covering a chamber that is around 1.5-2m deep.


Here's the conundrum:
Of the contractors we've spoken to, some have said that 'moving it won't be a problem', where-as others have implied that 'moving it will be costly'.

(In the background, we *did* get a build-over agreement from Thames Water).

So - for those who have seen a manhole in a typical suburban garden in London with base like the one illustrated - what do you think it's *actually* draining? Is it merely collecting water that's been drained from the nearby roof, or is it more significant than that. Will moving it be a challenge, or will it be relatively cheap/straightforward?

Looking forward to any helpful thoughts that people have.

Many thanks!
Oggy

[Tony McC]

I don't think anyone can tell you what it is draining without first familiarising themselves with the site and the surrounding area.

And the same goes for the cost of moving such a chamber - it could be a doddle; or it could be a right bloody nightmare! Again, it depends on the site and the suroundings.

In theory, the work is relatively straightforward, but what else is down there? What's the access like? Where can it be moved to? Is it problematic to stopper-up the inlets? How easy will it be to get shoring into place?

I'd look for a good groundworks contractor rather than a general builder or landscaper.