To be honest I do not understand your explanation of events. It sounds like Building Control are aware of the sewer so I cannot see how they could issue a completion certificate without a build over agreement from Severn Trent.If we somehow manage to get the completion certificate when the council come out, would that solve everything?
Even if Building Control did issue a completion certificate in error that would still leave the issue with Severn Trent unresolved and they could could take legal action.