We have a regard measuring 30cmx50cm and about 2m deep. On the house side, we have an outlet pipe out of which all of the waste water (sinks, kitchen included, showers, but not lavatories) comes.
On the street side of the regard, there is - maybe 10cm lower down - another hole which is the inlet to the town’s waste water system.
You will not be surprised to hear it does not conform 
The former occupants didn’t have any problems with it (they say) and were reluctant to change it - connecting the outlet to the inlet - for fear of creating a situation where a blockage might occur, because of the slight difference in height of inlet and outlet.
I wonder if it would be possible to reduce the risk of blockage if the pipe connecting outlet to inlet were in the form of a spiral - like a spiral staircase.
Any thoughts welcome.
If I understand correctly, you have effectively a holding tank into which is fed waste water, which then flows into mains drains?
I should expect the straightest, steepest run of time to be the least likely to block, because waste materials are less likely to build up. A slow-moving spiral is more likely to have problems because solids may be deposited and build up over time.
I would make sure that somewhere in the system there is an access port to allow for cleaning IF it should block up (again, easier to do on a straight run than one which curves).
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Good point about steeper being better.
Here’s a photo showing the outlet
It would be easy to keep it accessible
Who says it does not conform… and when was such a decision made??? That would be my initial line of questioning…
Whoever is in charge of Mains Drainage should be able to give you the full information on what (if anything) needs doing… possibly be best to be guided by them, rather than us… 
(Owners are often notified that something doesn’t conform in some way… but they can leave it… and it will be for the New Owners to put right if the property changes hands…
Is this what has happened to you ??? )
It looks quite good to me, but I think the issue may be related to the porosity of the block work. We have a largeish inspection hatch just as out waste water goes into the mains drainage, but the whole thing is a complete unit dropped in with a non porous lining and a metal hatch.
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