Roundtuit

Marmox blocks. I looked at them for ours, but in the end decided the benefit was neglible as we had continuous 40mm pir layer around the inside of the frame.

Split unit here. Yes, you can hear it if you're close, but I don't find it intrusive, it just adds to the 'heartbeat' of the house. Never noticed it whilst in bed or watching TV. That said, the installer that recommended it turned out to be a bit $hit, so I've no loyalty either way...

I've just paid £85 +vat for a service on the external unit, but as they're principally aircon engineers they don't do any more than a cursory glance over the indoor unit. I check/clean the water strainer on the indoor unit myself. I tried to get a plumber to service the uvc/expansion vessel, but they pretty much said its a waste of time and no-one bothers...

Hitachi Yutaki 16kw split system here, 5 years in. Can't fault it (other than a minor purge valve leak), but it is probably over-spec'd so doesn't work too hard. The only word of caution I'd add is to try and ensure that there's someone local to you that will service whatever you choose. After I 'delisted' my installer (for consistent reluctance to get his @rse round for annual servicing), I've found that the limited number of 'ASHP engineer' alternatives locally like to stick to their preferred brands and I've ended up using an aircon company for servicing.

Yes, the arch is supporting the brickwork above, and yes, it needs supporting or there's a significant risk of collapse by the looks of it. Depending on what the plan for it is, you could put a concrete lintel in, but that won't look pretty if you want exposed brickwork. Also, watch out for a soot-fall; I doubt it was swept before it was blocked up.

Don't see why not, then drop it into a waterless trap in the utility. The condensate pipe out of my unit is overflow sized; it's only taking a trickle at worst.

They frequently discharge into the house waste plumbing, so should have a trap on it.

I think I'd install sleepers a couple of feet away from the fence and plant some climbers/screening. Don't forget that if you're removing soil directly adjacent to the posts, you're effectively reducing the amount of post that's in the ground. Might be fine if they're in deep or you're well sheltered. Also, I'd carefully consider the wisdom of building over a manhole with the pizza oven (if that's what it is)... there's some universal law that says you'll need to rod the drains before the first pizza is cooked 😆

@RobertW Apologies if I've missed it, but what are the doors? Upvc or timber?

I think I'd want the cladding cut shorter to leave a 10mm gap to let the end grain dry after rain and reduce the risk of capillary action. As above; wall build details would be useful.

Thinking about it, it's not just the plinth bricks. You'll also need wider foundations and the block work behind the facing bricks, as you're effectively building a double width wall up to plinth level.

Plinth bricks can be expensive. From memory ours added >£4k plus labour, but it depends how many corners you have, and whether there are 1 course or two. A cheaper alternative for us would have been to use some non-matching engineering plinth bricks, but I didn't like the contrast. It does give the house some additional character and a traditional look though.

Shouldn't be a problem, but if the house has been designed without mvhr, then I suspect you've got a combination of trickle vents on windows and extractor fans planned, and are not very air-tight. The benefit of heat recovery will therefore be minimal, so maybe just consider an extractor fan on a humidity sensor for the utility room/kitchen/bathroom?

I did ours with osmo polyx oil after hanging. It's a bit of a faff working around hinges, but you can do both sides in one hit and don't have find space/time to lay them out/turn over for each coat. Very happy with the finish (rub down with a fine sanding sponge before and after coats), and no signs of any problem due to not treating the bits under the hinges or under the bottom of the door. I'd do the same again definitely.

Induction hob definitely. Gas bbq for emergency and steak.