I am minded - not yet decided since Carlise is half a day's travel each way - to take a look myself so intrigued am I with this problem.
My suspicions have turned to the smart meter. it contains a contactor which isolates the incoming mains in the event of tampering, end of credit, remote supplied command or loss of incoming mains power. The working hypothesis is that there are very brief complete interruptions in mains power to the output terminals of this smart meter caused by some sort of fault associated with this relay or its control including the meter enclosures anti-tamper circuitry. When there is a a very brief interruption in 240Vac when some non-trivial current is being supplied as would be the case when you and /or your wife are using electrical equipment or indeed when fridges and freezers start or run - then the transient currents which flow briefly after power returns to normal causes very high but short duration pulses of magnetic flux in the short circuit solenoid of the mcbs. These pulses of high current high magnetic flux are interpreted by those circuit breakers passing current as short circuits and thus they trip. Oddly but not physically impossible the sunhouse mcb trips even when it is only connected to the busbar but nothing to its output because the high pulse of magnetic flux(es) from adjacent mcbs link its own short circuit solenoid causing it to trip.
These events are initiated and noticeable only when non-trivial currents are flowing thus during gaming, cooking and the use of high power white appliances.
It is still not certain even though the dno have been and tested(? ) that there is no external fault with the distribution network your home connects to noting it is a PME supply. If it was a street long problem others would be complaining - and they are not - so if there was a supply defect then more probably it would be in the feed to your home.
Is your street fed by overhead lines?
Would you please load up the ring circuits and your sunhouse too (so reconnect the fed to it to the B40A mcb) and then take a rolling pin or hammer and tap the smart meter in a number of places a number of times. Obviously don't beat the living daylights out of it as much as most of us would quire understandably want to at 50p a kWh!
After this, please take some small plastic clear freezer bags and aluminium foil. Using an mcb as a template cut out two pieces of ally foil shaped to both sides of the mcb. Place these each in a small plastic freezer bag. Interposed these insulated ally sheets between the sides of the mcbs which usually trip. Wait and see what transpires. Obviously take great care that the ally is inside the plastic and does not touch the busbar. No shame if you think this is too risky for you to do. You really do have to be very careful. I only suggest it because you are a qualified electrician. You are making a Faraday shield. See
Faraday cage - Wikipedia - https://en.wikipedia.org/wiki/Faraday_cage
Also wiggle the meter tails to see if their movement causes the trip.
I recall a smart problem in Birmingham when its radio transmissions interfered with the electronics of the rcd causing it to trip - but your rcd does not trip so we will keep that on the back burner for now.
Is the meter cupboard metal or plastic including the door? Is the mobile phone signal strong where you live? How old is the smart meter in the cabinet?