Benching for slipper bend

[Hobby builder]

I have an old brick inspection chamber with a single channel running through it with benching on each side, that carries foul waste. I want to run a new kitchen waste into it. I would prefer not to disturb the existing channel (I know from experience that when something is working it is usually best to leave it that way), so was planning to run a new clay pipe in at 45 degrees to the main flow, with a branch channel sitting just on top of the existing main channel (a stepped slipper bend). I have cut a section of the existing benching out ready for the new branch channel, and am unsure of how to proceed.

Given that the new branch channel piece ends in a half-round section, should I build up the benching around the sides of the branch channel (which would of course need to be higher than the existing benching as the new channel sits on the edges of the existing one) which would give raised benching each side of the branch channel end OR should I cut the branch channel end so that it effectively follows the curve of the existing benching and just fill in the gaps in the benching so that it is the same curvature as before (as if I had filed the branch channel end to match the curve of the existing benching)?

All suggestions gratefully received...

[seanandruby]

it depends on who is inspecting it and what they will let you get away with. you cant just cut pieces out of main channel and hope to bodge it up. you will either have to fit a flush side access channel, or, if inspector allows it stepped.
im thinking as it is kitchen waste he may let you do the latter. make sure the benching is raised up at the sides then angled over smoothly. dont leave any flat parts, or, snag edges. its not as hard as you may think. you can get the grano mix qntities off the index. remember to give it a good clean first with jeyes fluid or similar and where ppe. good luck.