In answer to P's question, yes, some of the pipes are being rerouted - there is a pipe leading from the roof gutter, which will need to be rerouted completely, as it was clashing with our wall. Currently, it's going nowhere, but it will be rerouted on the outside. Other pipes are staying more or less where they were.
The big junction and manhole which is currently beside our party wall will be moved to just outside the extension, in front of our shed door - though how it will get through the wall, I'm not quite sure. No doubt they've got it all figured out and it will become clear in due course!
We discussed the foul pipe coming down from the main bathroom upstairs with Kevin today. The original plan was to reroute this between the ceiling and roof in the extension, to come out the side of the house on the other side. But as we have now decided to go for a vaulted ceiling, there is no room for this, so we're going to keep this pipe where it is, and box it in(and insulate it well so no unpleasant sounds are heard when we flush the loo in the main upstairs bathroom)
The main sewage pipes going across our garden have now been covered over and will never be heard of again, hopefully!
This morning, Kevin had to do a bit more digging to confirm that the damp patch we found on Friday night at the back of the house was not a leaking pipe, but just a harmless bit of ground water.
There was plenty of calculations and discussions today, about pipes, walls, radiators. The engineer was called on site to discuss pillars, steels, beams and RJs (jargon jargon). It's all clear now, and everybody is happy. I think!