Thanks all for the suggestions. None appeal and tick all the boxes as microcement does for us. We are now thinking that if we do get cracking, we will embrace it, japanese Fukumoto style, and fill it with gold resin. This might be a terrible idea, but haven't been able to find another solution we are as happy with

People are sometimes concerned about recent movement or recent underpinning in case it moves again. But you should be vigilant rather than concerned about 13 year old movement. A surveyor should help identify any movement since 2007. Underpinning basically means the foundations were made deeper and stronger. The solicitor will find out more information from the seller- ideally the job was done through the buildings insurer and is guaranteed. However guarantees don’t last for ever so it’s not imperative it has a guarantee- it’s important that the information from the solicitor is enough to satisfy buildings insurers (you may need to use an insurance broker to bypass all the insurers who don’t get involved with this kind of thing). You may also want to satisfy yourself how costs may be higher for this house.... for instance your insurance might cost more.... any extension might need deeper more expensive foundations and so the price you pay might need to reflect this.

Storage heater idea seems to have merit. This is what a normal one looks like inside... Just please make sure the metal cables are not live electrical cables! I doubt the walls have any structural function as they are sitting on top of the floorboards and one of them seems to be in front of the wood panelling. They may have been used to cover up a damp problem? If in doubt call in a local structural engineer and get them to sign off removal of the walls.

Even when a sub floor has movement it would normally be the case that the overboarding would disguise any movement. I had LVT put in a couple of the offices at work and I know the boards underneath were poorly screwed down. But there seems to be no movement of the LVT. I would be optimistic that the floor installers can sort the problem.