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It's hard to go much higher than a 20% hydrogen blend as that's what will work with the millions of old gas boilers that will still be around for decades to come. There's only one gas pipe down your street, you can't pipe a richer hydrogen mix down to just the newer boilers because it's not safe to fuel all the older boilers. (Most hydrogen is produced from gas anyway!)

@ETC a protected stairway is described in approved doc B 2.5.a and may apply to a two storey design such as this as described in section 2.2.b, being an alternative to the escape windows described by 2.2.a

With a heat pump for water and heating and being happy with an electric hob (for aesthetics as much as how they cook - we'll miss a giant burner for a wok) we're not bothering with gas. As @JohnMo says, why pay the standing charge and more significantly, why pay the cost of getting a new gas connection in the first place - probably £2k and upwards? If you must have a gas hob then maybe, but I think in an efficient new build property it's hard to make much argument to use gas for heating over an ASHP.

Probably that your bedrooms open directly to the landing and staircase, leading directly to an exit downstairs, and that area is protected by fire doors etc. That's how you'd escape, not through the windows

I wonder if they've not laid them particularly straight and then knocked corners off to get them to sit flat? As @Mattg4321 says I'd expect an eaves tray no membrane showing, and I doubt that gutter's clipped in at the back. I suspect if nothing more is broken underneath they'll do the job. At the least get a hosepipe on it to check for obvious issues

You could ask their PWS in writing/email to clarify their comments whether the wall will be at the boundary and its capping overhang, I suspect they'll be less flippant then. This at least is somewhat promising. Honestly I think it's worth trying this approach again. You may want to read about this dispute and then have a chat with your neighbours whether it's in either of your interests to get into a formal dispute and court action over this. Put aside your concerns about their son as the builder in favour of asking for a small compromise to set the wall back slightly to avoid a dispute that costs you both money and devalues both properties because you're in a formal dispute, regardless of who is in the right.

The free online version of Sketchup is still quite capable and definitely worth giving a spin as you'll already be familiar with the UI. For basic modelling it does quite a lot but is missing third party plugins, the render quality is limited, but that doesn't sound like a deal breaker for you. You can import any existing models produced on desktop. I found the old free desktop app crashes a lot on modern Macs.

As mentioned earlier, the smaller your build, the more expensive per m2 as some costs are largely fixed. Is there any possibility of making the build 'one and a half' storey? Lose some space for stairs but gain another 15-20m2 upstairs, without making the profile of the property all that much more imposing and at pretty low extra cost.

Do you already have a copy of the planning officer's report? It didn't sound like you know their recommendation yet, so chase that. Your prior approval is a big plus and a recommendation for approval would be even more so. Have you read your local council's planning committee protocol? What are their rules for who can speak and do you need to register for this ahead of time? If there's a chance for a supporter to speak as well as you so much the better - could your planning consultant speak in support so they can emphasise the sound policy grounds for approval. Have you lobbied your local councillor for their views? We invited ours over to discuss the plans a few days before.

How many times did you fill and refill the holes when saturating? Rule out the chalk hole, then the big slowdown between tests for the second hole suggests that ground may not have been saturated enough to give a reliable reading. I'd fill and drain that a couple more times and then repeat the tests over a day or two

I thought rubble filled was still allowed, but has to be about 3x larger than crates.

Is making the island 100mm deeper an option? You'll have end panels covering the sides of the back to back cupboards. Otherwise change the 2*900 wide cupboards at the back to 3*600 so you only lose space in the middle one?

Perhaps you could explore a 'blue roof' - could the tank go on the garage roof maybe?

I think there's a lot to be said for this. While boarding out can be done very well it can also be done very badly for airtightness without that being obvious and post plaster is an expensive time to try and make improvements. Airtightness is an area it helps to be hands on to ensure it's done well, but if you can't give the time then at least aerobarrier sounds a reliable way of outsourcing for a good result @zagaboo I agree Plasmore have good compromise options. We used their Fibolite which are 0.28 and still readily take fixings, direct nailing compared to thermalite

I dislike Thermalites too. If it helps, for a new build the limiting value is actually only 0.26 (table 4.1 of Part L) which gives you flexibility to use a different block, as long as you make adjustments to improve efficiency elsewhere - it may be easy to add 10-20mm of PIR to your roof or floor, say, to achieve the same SAP score.