I just found this new publication, which explains the problems of thermal bypass and how to avoid them.
Thanks for the imformation, its going to take week,weeks to read all of it. A lot of lmformation about where we went wrong in past. My bigest change will be service ducts.
My own house has 2 service ducts. One only goes up to the first floor but the other penetrates into the loft. Even 17 years ago I knew about the chimney effect in vertical ducts and put baffles at each ceiling level but it never occurred to me that it needed to be airtight at all points. It looks a bit like the party wall illustration but without the airtight membrane.