I've downloaded half hourly from Octopus, I think they gave it in gas m3 though

Yes, most meters (including all Smart Meters) record gas consumption in cubic metres (m³)

To convert to kWh...

Cubic meters (m³) used x calorific value x Gas Correction factor (1.02264) ÷ conversion factor (3.6) = kWh

The calorific value (CV) of gas varies slowly across the UK daily, and there are 69 daily sample points on the network.
The CV used by your energy supplier is printed on your bill.

It will be figure around 39 MJ/m³ (MegaJoules per cubic metre)

You can obtain the CV for all sample points across any range of dates via the Gas Data Portal.

CV data comes under Supplies - Daily Actuals (physical) - Calorific Value
then choose the date range before selecting View Data at the foot of the page.

I'll dig deeper into a fan coil radiator. 

Have a look at Jaga Strada units.  We have them everywhere and they work well.  

You can set low  medium or high fan speeds at a given flow rate, which changes the heat output (and presumably the DT) without changing the flow rate.  We leave ours set to the lowest speed, which is very quiet. 

How did they adjust the fabric sheet, by just ensuring the total output from the radiators in the house balanced the total heat loss/energy usage even if room by room it didn't match? 

Pretty much that, basically (1) make sure that the fabric model represents the actual fabric and (2) adjust the ACH to fit measured loss.

Here is the data I used.  This is 2 years worth but 1 would have sufficed

I can't be certain of the exact schedule as I have constantly tweaked start times/temperatures slightly. Basically we run it at constant temperature approximately 18.4 set point now and on from ~6am till 8pm with a set back of 15 or 16. 

There may have been points where we kept this at 17.5 or something through the day, or had it come on an hour later in the morning depending on the kids wake up time! 

How did they adjust the fabric sheet, by just ensuring the total output from the radiators in the house balanced the total heat loss/energy usage even if room by room it didn't match? 

My boiler is a 2005 condensing boiler, so while theoretically reasonably good efficiency the previous owner did not get it serviced. 

Aug-Dec 2023 was 3135 kWh, 2024 was 6,750 kWh and 2025 so far is 443 kWh. I guess that's in line with a 7 kW heat loss at 21 degrees rather than the below 6 I see to keep it at 18. 

I have reached out to Adia, hopefully they find my project interesting enough to pick up during their development stages! 

This room is an open plan kitchen/diner/lounge, I'll dig deeper into a fan coil radiator. 

I remember thats where you started (conservatism in heat loss).

What do you actually know in total about heat loss.  Do you have any consumption figures for past years.  FWIW my house was rated by two surveyors at 16kW (full three hour surveys in each case).  Doing the calculations properly using MCS assumptions gives 10.5kW.  Its actually 7kW, so, if my house is anything to go by, you are right to be cautious about loss figures.  I collected a fair few figures together to size my installation.

One final option which might suit you is adia thermal.  They fit meters and special heads on the TRVs allowing them to measure house and room loss, and optimise the flow settings.  One of their ideas is that you do the ashp installation in 2 stages, ASHP first then rads, rads only after you have collected lots of measurements to determine which rads actually matter.  

i don’t think my Mrs will ever sign off on taking everything out of the downstairs to do it….kitchen out, toilet out, more new flooring, remove fireplace…then put kitchen and toilet back again

it’s tempting though…

Nor are they particularly expensive.
Underground drainage components are pretty cheap when you look at the overall renovation work.
A 3m length of 110mm diameter pipe is about £14, and an inspection chamber, risers and cover can be bought for less than £100.

There's nothing to prevent you doing (the majority of) the work yourself...
... and you certainly don't want to be paying a qualified tradesman just to dig a hole!

I have a 'live' sewer running beneath my extension, for which I took advice from the local Building Control officers (who aren't actually meant to do so!).

I placed new inspection chambers at each end of the run, and ensured that there were no joins/elbows actually beneath the floor.

Here's the downhill side, where you can see the cover of the inspection chamber, and a couple of spare 'riser' rings.
These allow me to set the height of the concrete cover once I've worked out where the finished ground level will be.

The building trade may take short-cuts and fail to properly install stuff,
but self-builders don't.

We're hardly likely to compromise the standards on our own house!

Building Control surveyors know this.
Once they've assessed our work on the first of the 14 main check-points, it's unlikely they'll bother to attend on-site again!

This Forum will continue to advise you as you wish.
Just keep us informed of what you find.... preferably with an accompanying photo.

@editor and I can split this out into a separate topic, so please ignore the Topic Title and keep posting.

the issue with this is…on the earlier drawings, if all those drains and gulley run into that one manhole, which is now under my concrete floor and also under tiles…how do I access any of that to add inspection chambers, and where would I put them?

I can’t think how I’d access them from outside of this either

the only thing I can think of…

…the garage annex had an en suite installed. The toilet, sink and shower wastes all run straight out of the wall and into the ground outside of the house…just outside the utility room door. I imagine they also run to the same manhole.

maybe they could also be used for inspection if ever required?

Just a couple of observations

1. Sadly I would say the standard of work is exactly what one expects, this is either the building trade and/or quite possibly in part DiY, and the use of the extension is/was somewhat subsidiary to the 'main' house; just from casual observation poor construction seems to occur frequently in these circumstances.

2. Sockets at the level depicted are very common, I would say (from casual observation) more common that sockets higher up (just because of the age of our housing stock).  I don't know when it became a requirement to locate them higher and to what extent that requirement applied to extensions.

3. Combined drainage, where foul and surface water route to same drain, were the norm until (I think) the 1970s.  I dont doubt that, where combined drainage exists, new surface water drainage eg from an extension gets connected in whether or not it should be.

4. As @transparent says the integrity of the foul drainage is paramount and should definitely be checked!

hi, again apologies for the delayed reply

1. Good guesses, I expect you’re quite accurate, we are ready to try and make it more inhabited hence the desire to indulge it and heat it properly! I hate jobs that aren’t done ‘properly’ with cut corners, unfortunately that’s what this house is and at the risk of generalising, I’ve struggled to find a trade in the south so far who isn’t part of that problem

2. We changed all the skirts to more modern ones…didn’t realise at the time that the sockets were so low and on a couple we had to even cut them sockets into the skirts which looks awful. One day I’ll move the sockets up and fix it but you move one, you’ve got to move them all…

3. It’s a 1972 house so I reckon it could be combined. My only real doubt is that the river gamble runs past us and into 2 large ponds in the village, which are also filled by local rainwater…or so I believe. That would imply you’d need a separate fowl water system. Maybe I’ll look more into that

4. I will look. I’m recently out of hospital so heavy lifting isn’t on the agenda for now but I’ll at least have a look tomorrow and order some drain keys if I think I need them - we replaced the cover with a block paved one so it’ll be pretty heavy