Mr Punter

It sounds like a combination of poor fabrication, poor install and poor quality sealed units. I don't know how the profiles perform. I think they are Deceuninck.

Yes. Stop worrying and move on.

If the brickwork faces south west and is a large wall, a fair amount of water will get in. If a stepped tray is difficult, do it horizontal and a coat of Stormdry on the remaining triangle of brickwork for belt and braces.

The membrane should be fine not faulty. As you have sarking and counter battens there is no need for any droop. As @Nickfromwales says you could have laid the membrane flat on the sarking but no matter. I suspect the error is where the membrane meets the top of the Velux and the flashing / membrane has not been cut as shown and installed correctly.

The layout is really compromised as can be the case with bungalows. The extensions have not improved matters. If the current setup does not give you enough space I would look elsewhere as you would have to go to considerable expense to make your required silk purse.

If this is just a connie erected on a single skin dwarf wall you may be better to demolish and build a proper extension, as the roof, glazing and walls will probably not meet regs.

Interesting to see they have used what looks like a mix of 30mm eps board and blown graphite eps beads to insulate the wall. I would guess the car is an auto or EV.

I think that will be a struggle now the joists are fitted. You would need to unbolt and remove the ledger which may not be practical.

Terram and crushed concrete, finished with a sit-on vibrating roller.

You may be far better to get it excluded and get the policy in place.

The only concern with FMB would be if they were claiming to be members but were not. You have done decent research. Did you get 3 quotes? If not, make some calls. It is going to cost you a lot of money, so make sure you choose someone who is good and you get along with. You don't need to be mates, but there may be potential differences that will need resolving.

I have used a chop saw. One of the Evolution multi material ones. Goggles are a must.

Have a look at D-Line. They claim to be softer material and may work for you. https://www.screwfix.com/p/d-line-tsb35w-2-2-gang-surface-pattress-box-35mm-2-pack/143cy#product_additional_details_container

OK he did say But I think you are right.

Sounds like your warranty company are being dickheads. Is this Protek?