Mr Punter

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.

+1. For a new dwelling a full plans submission is needed. Building notice is OK for small works only.

Looks OK to me. I would rather a load of mass concrete and some tile adhesive rather than a mortar bed as the clay may shrink and heave which could loosen the tiles, but it is built now. Maybe keep a few tiles in the garage in case? Some drainage through the wall would be good.

Learning by doing. You will ace this next time!

Have you got a section detail showing the foundation, slab, insulation, DPM, DPC, screed and FFL?

I think he is a Scot, so no words minced!

The way to work this roughly would be assume 20% of your heat losses are through the roof and assume your improvements could halve this, so take 10% of your annual heating bill of, say, £2,000 and with a 15 year payback this would allow capital expenditure of £3,000. This is the minimum BC would be looking at for part L improvements. They may allow you to make the improvements elsewhere if they are not possible in the room you are working on.

Have you applied for Building Regs and paid the fee? If not, don't bother.

You often will just need basic information for an Initial Notice, then they will do a plan check and come back with any queries. Main points are foundations, structure, fire safety and drainage.

They often set up tripods to set the levels. Is this 2 guys working and 1 shouting at them?

You might want to try Sticky Stuff Remover or similar.

A sparky might comment but I think the blue and brown wires need more protection - like trunking.

If you are renovating it is a decent opportunity to improve the energy efficiency of the building by increasing insulation. The minimum fallback is spend the amount of money that would be saved in 15 years on the improvements. You may be able to claim there is no practical way to improve the roof insulation that would provide such a payback. Hopefully your heating bills are low, and if so, happy days!