@carpenterstation  

When changing settings in the service menu you often need to shutdown the system first, so if it's running it'll ask you if it's okay to shutdown and afterwards it displays the settings you can change.

the only option I can find on the FTC under operational settings in the service menu is “System shutdown” ? I must be looking in the wrong place ?

If you don't want to use summer mode (https://renewableheatinghub.co.uk/forums/postid/45608/) then you could slide the zone set temperature sliders to their minimums in MELCloud to turn off heating without turning off DHW. 

@robs Is this only in the newer controllers I don't have that option on my FTC5

The FTC6 and FTC7 both have summer mode. 

Yes that needs to change but its not going to be a quick process, this isn't a Mitsubishi specific issue its an industry issue.

You could sign up for Melpump its a web app that gives you some further control.  The first month is free then annual subscription is currently £26 you also need an additional wifi adaptor that is £30

This would allow you to disable the heating from your phone and leave the hot water active

https://melpump.com/

I have an “actions”, “All off” option top right of the “building list” page but that turns off Hot water too, and there is an “on / off” option on the building itself top left. But again this turns off hot water too. See attached? Is this normal?

Please correct me if I'm wrong here, but isn't the temperature difference between each pair of ports a function of the difference in flow rate on the two sides of the buffer? 

Yes

So what I'd think from this quick thought experiment:
 - minimal temp difference between each pair of flows is exactly where flow rates are equal.
 - it's better to be on the higher on the secondary side than the primary, than vice versa.

Yes to the first

No to the second (I think) as this results in cold return water being swept up into the flow, reducing the flow temp to emitters for any given flow temp from the HP.  Thus the HP COP is lower (since the required flow temp to the emitters is determined by the output you need from them).

Given this is presumably out of warranty and if you fancy some DiY plumbing you might want to consider bypassing the buffer (turn it into a two port volumiser on either the flow or return) and secondary pumps altogether.  With the right combination of shut off valves this can be done reversibly.  Nice summer project!