Hi mate - our scenario was the same. Dealing with Yorkshire Water though, they wouldn’t permit a build over, and wanted a MH/IC along the alignment of the original sewer inside our boundary. Unfortunately, we had to dog leg the extension at the outflow end to cater for an IC and tie in from our stack - not ideal but it works. As for replacing existing pipe work, if you’re at it and have the plant on site, I’d definitely replace any vc pipe work with new plastic, boundary to boundary. Old vc pipes crack over time and it’s additional piece of mind to know everything under your gf slab is in good condition with better flow velocity. Good luck with the build