Hmm, a messy situation indeed, and one to tax even BM's brain.
A couple of initial observations:
Are you confirming that there is an easement built into your LR document describing the rights for the sewer owner, tied to the LR plan which shows the position of the sewer in a place where it isn't?
(Nit-picking point) This isn't a planning issue, nor is it a technical breach of Building Regs - because the sewer has probably been built to the right standard (allegedly) - it has become a problem because the Developer will have covenanted with the Local Authority that the sewer will be adopted and now finds that difficult because of the distance thing. It is the Water Company has sets the 'rules' that say a building can't be closer to a sewer than 3 metres - not the Sec of State or the local authority.
What do you want out of this? Not being funny, but the options are probably:
1) Big compensation because of the loss of an amenity (your plan for a conservatory)
2) Force the Developer into a situation where the sewer can't be adopted - hence it becomes the residents liability - for some compensation to each of the owners who feed into this sewer and end up with a small future liability ?
3) Move out with all your cost paid for? (probably unrealistic)
4) ?????? (I can't think what else you can do.)
Surely 1) is the only realistic possibility? - it just depends on how much.