Mr Punter

Sometimes I have had the sockets on a flex under the units. Quite a few appliances have moulded plugs and it may be that any warranty can be voided if you cut them off. It is not simple to anticipate what is needed. I have had induction hobs needing hard wiring on a separate circuit where others are plug in 13A. I like under cupboard lights to be switched from an extra gang on the main light switch, which can also be a challenge.

Leica are a very good brand but IMO too expensive. The Chinese stuff is so cheap it is hard to say no.

You don't need to be qualified to build a house in the UK. ICF is fairly easy DIY. It would be more impressive if you built in brick and block. No requirement for Gas Safe if it is just for you but would be my preferred option.

The Dritherm 32 slabs are easy for the bricklayer to fit, easy for wall ties, decent u-value, don't make a mess if the wall gets drilled and easy to clean up mortar droppings. EPS beads are good too but you said you are not keen on them.

Proper rough job. It would still be rough without the rigid insulation, which elevates it to farcically bad.

Gas Safe plumber would be my answer.

Bear in mind that lots of EWI has been a disaster recently. Lots of mould, damp and ventilation issues, so this needs to be done meticulously, especially mixing EWI and IWI.

Try it on a small section.

Yes. Look up "Leaky Condo" and see the consequences of omitting a drained vented cavity on timber frame.

Dritherm 32 cavity slabs.

Can you space them at 300mm (4 bricks) vertically, staggered each side of the pier, 100mm in? That would give you about 6-8 ties.

I think you may find it tricky to do much as you will need to drill / hammer to get any ties in and this will disturb the brickwork further. Maybe helical ties would work?

I would replace any completely knackered bricks and coat the chimney with StormDry.

Agreed. Better is a render carrier board battened off the structure to leave minimum 25mm drained and vented cavity.

I think it may work well with either internal pir or insulated plasterboard, so move the 50mm celotex inside. It follows the rule of thumb of having the most insulating and vapour impermeable materials inboard.