It does sound like a bells and whistles job. I dare say the house is worth over a million quid though pretty high spec. Mine will deffo be diy, last one was. I won't be having the plaster in vents.
Cool. What do you have 2 holes through the wall and they just connect into these? Neat solution. I went through roof tiles at my last place. How did you get on with your SAP with the titon units did you buy these before the design stage or did you change your as built SAP to allow for them?