Oldbutnotdead - thank you very much for your time. Chortled at your reply - "tropical 20 deg"... wish my wife thought 20 deg was tropical! She unfortunately likes her skin to blister in the shower, and still wears a jumper on as 30 deg summers day! Now to your questions:
1. Yes we do have a HW cylinder - its by the boiler (i omitted it to try and keep the diagram simple)
2. Yes we do have a bypass valve at the Boiler - didnt know what one looked like before, but now i do, and i can see it between the flow and return
3. Lol - yes, we have a few more rads upstairs (again, trying to simplify!)
4. Yes, whenever we have had work done, or a carpet up i have insulated, but it is so difficult. We live in a very old house (single glazing, no cav walls, minimal insulation), with long runs, and rennovating bit by bit, but i still think there is masses of heat lost warming voids under the floor!
5. Re smart TRVs - i will continue to have an open mind. Its just if i have 1 rad coming on - i fear it wont be as efficient in a modern insulated home, as once all the pipes are heated up, you may as well be running all the radiators, because of the length of the runs.
Is it ok for the new flow and return to t-off per the drawing above? And is it a problem having 4 dead ends where the existing pipes get cut and capped? Thank you again for your time and thoughts OBND! Dave