UPDATE 13.11.23
Insurance assessor is attending the house this week to assess the damage from the subsidence caused by the heatwave. Flo and I cannot get there but we have our friendly builder / decorator meeting him so they can discuss on a professional level what is what.
Will update further when we know more…
C
Good luck.
Likewise.
On se tient les pouces 
Easier than holding one’s breath. 
Thank you everyone.
Insurance assessor visited the house on Monday.
Has followed up with questions by email :
When were the external walls last painted? (what that has to do with subsidence is anybody’s guess…)
Do we have a septic tank or are we on mains drainage? (Mains thank goodness - but no idea why he wants to know that).
Have we had problems with the drains recently? (I can only assume that either he thinks the drains are leaking and causing ‘wash-out’ of the soil OR, if we are having problems, then the subsidence has cracked the drains as well).
Are the oaks along the nearby boundary on our land? (They are - and obviously so as the fence is beyond them).
I know the oaks have contributed to the subsidence as they are probably the worst tree species for sucking water out of the ground in drought conditions - but they have been there a damn sight longer than the house and that was built in 1972.
The saga continues…
Conifers are worse.
I presume the house is in France.
Having built many houses in the UK and worked with insurance companies on subsidence related claims I am well aware of what can cause subsidence.
A house built in 1972 would not have have taken account of foundation depths to get below the root table of nearby trees whereas today it is accounted for.
Your problem may well be inferior foundations close to large trees.
Our expert came to look at the subsidence in February.
When he got out of the car he was visibly unwell, couldn’t stand for long.
He couldn’t connect to our wifi, nobody else has ever had a problem.
He made paper notes, inspected, with difficulty, the floor and walls and decided it wasn’t structural and that movement would be ongoing therefore not worth any work.
He then tried to get into our car to leave!
Two days ago we received a phone call, the expert is in hospital and has not done our report.
We therefore have another expert booked in for June!
Could be interesting.
TBH I feel sorry for him. He was trying to earn a living and perhaps do a job he loved, anyone can fall ill, my heart goes out to him.
A second expert sounds more promising from your description, fingers crossed for you. Using a soft lime mortar if its not a structural issue would at least stabilise the gap. NHL2 rather than the harder 3.5 to see less cracking in future maybe.
We did too. It was painful to see and we were more worried about him than the subsidence.
I expressed our well wishes in our email to confirm the new rdv.
I’ll be watering my foundations this summer. 
Sadly wasn’t just your building subsiding, your expert also 