Just been on the phone to the solicitor, she wants to contact Severn Trent herself as we've not been given any info. She is writing to the other solicitor to request permission to do so.
As for the drain, something just clicked in my head. The access hatch for the public sewer is at the side of the extension, the public sewer runs parallel to the houses (according to STs map anyway), which means the extansion is over a public sewer not just my own feed into it. I suspect this has greater implications than it just being my own bit of sewer, as lots of other peoples waste is flowing through it?
Have to say my solicitor is being great on this, can't fault her. She's chatted to another surveyor about it, and is calling mine now to find his opinion. I'm still getting a CCTV survey (tuesday) regardless, but I need consent from ST to have a dwelling built over their sewer and that does not appear to have been done.