How long do you think you'll stay in the house by the way? its a huge amount of work to do on a property if you will move in a few years, although will obviously help the next owners.
Sure. We'll probably stay in this house for at least another 5 years, maybe much longer. It depends where work / life take us

I have asked about whether or not our refurbished living room would show up on an Energy Performance Certificate when we come to sell the house. Some people say it will show up. Other say the SAP software isn't sophisticated enough to be able to enter different thermal performance info for each room. Part of my enthusiasm for getting Building Control involved was to maximise the chances that our work would show up on our EPC.
when you did the first half of the wall, did you space the wood to be the width of the celotex or did you cut the celotex to fit the gaps between the wood? or is that what you discovered you should have done when you say at the end to keep things simple?
On both sides of the room the studs are at 600mm centres. The big difference was that on the first side of the room I used 50x50mm studs with 50mm Celotex between the studs and 30mm over the studs so yes, I had to cut the 50mm Celotex into 550mm strips to fit between the studs (it's supplied as 2400x1200mm sheets). I found it surprisingly difficult to get a perfect fit (my cutting wasn't perfectly straight along the entire 2.4m length of the Celotex plus the studs are a little bendy) so I had to liberally apply expandable foam to keep fill the gaps between the studs and the Celotex (gaps would have allowed cold air to pass from the ventilated cavity between the two layers of Celotex).
On the 2nd side of the room, I just threw the Celotex over the studs, no cutting required (expect at the edges of the wall, of course).
please post again in January with the effect.
Sure, will do.
ps ever think you should have dug down more and built a basement or huge rainwater tank? (joking)
I did think about putting a tank down there, yes! But I quickly went off that idea for a bunch of reasons!