New inspection chambers- what size?

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Evening all,hopefully this is in the right place (there isn't a forum for poo pipe).

This will be tricky without drawing it but here goes.

House is detached, about 15 metres away from nearest house at the gable end wall that matters. Existing drainage goes (from 2 gullies and a stack at the back of the house) to a rectangular inspection chamber at the rear corner of the house about 300mm away from the gable end and back walls. Flow exits this (450mm deep) inspection chamber via an interceptor trap (with bung, in place!) and heads to the street at an as yet unknown gradient parallel to and not very far from the gable end wall. Pipework is all salt glaze and not shared (mine is the only house on this branch)

Problem- CCTV survey shows some damage to this pipe- haven't had the precise mapping back from them yet but there is evidence of previous subsidence to that wall. Also one of the mods to the house involves an ensuite which will need an additional stack at the front of the house linking to the run to the sewer.

Plan- run new (plastic) drains under the drive about 3m away from the gable end wall and link to the existing pipework as near the street as possible (house is set back from street by about 5 metres).

Questions- Part H has me confused. I suspect that the salt glaze will be about a metre down near the kerb- do I HAVE to have (is it wise to put in) 750 x 675 manholes or will a 450mm dia inspection chamber be sufficient? There will be quite a few junctions in the proposed scheme (4 maybe 5)- the plastic 450s are cheap and easy, the 750 x 675 will be neither.

I'll be talking to building control about this but my friendly inspector has retired and I keep getting the pedant on the phone so I'd appreciate knowing what I have to do before I talk to him.
 
300 mm mini-chambers OK up to max. invert depth of 600 mm

450 chambers OK to 3000 mm (with restricted access cover) or 1200 with a standard cover
 
Cheers all, that was my reading of Part H as well, good to get confirmation. Drawings- yes, AutoCad is still sulking about Windows 10 and I didn't want to spend half the night doing a shoddy sketch in Paint. And forgot to add- once I've put the new pipework etc in, the old will be getting filled with a very liquid concrete mix :)
 
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