Well after a bit more prodding and poking something I have done has improved things.
Not sure for certain but I removed the cable from the flow meter, causing the FTC to error, then plugged it back in and reset at the main controller.
Now it is giving the correct value, maybe done 1-2 l/m on what the OSM flow rate is.
I’ll keep an eye on it but leave it for a bit. There is some cold weather coming up so if my bad DHW cycles are going to happen again it is likely then. I can then get Mitsubishi to take a look again.
P.S. Annoyingly the Mitsubishi person looking at my data yesterday on MelCloud means my pymelcloud script can’t connect as it is being throttled.