So once I've set up my water law (which I've more or less done) then I could schedule the hive to be (say) 2C cooler at night than the desired 19C - which would have the effect of limiting the pump.  Is this correct?  Most people say "don't use a hive!"

I'm not familiar with Hive thermostats/controllers, but as @jamespa says, not all external thermostats work well with heat pumps. Mine has 3 different modes, so I just use the simple on/off configuration rather than one of the clever modes designed to work with boilers.

Again, as @jamespa says, you have two choices - you can either set a lower room temp (say 18C) which will switch off the heat pump until the house cools and calls for heat again. Or you can manually adjust the flow temperature (as set by WL) by +/- 5C on the front screen of the Samsung controller. Once you have your WL correctly set up, you could try setting this to -2C overnight (before bed) and turning it back to 0C when you get up. Unfortunately there is no easy way to schedule this, and finding the correct offset may take a few attempts. Manually setting the flow temperature to run 2C cooler (or whatever value works for you) should allow the house to cool a little overnight.

I have an EPH CP4 programmable wireless thermostat (recommended if you want a 3rd party thermostat that is convenient to use and ASHP compatible) which allows times and temperatures to be programmed which works great for the first approach as we're not really able to use the second approach as my heat pump puts out too much heat for my property overnight, even when the flow temps are turned right back to minimum, so the thermostat hard on/off is the only real option for us (with an oversized heat pump) to maintain an 18C overnight temperature. If you can achieve the desired set back by manually reducing the flow temp overnight, then that becomes an option for you.

No the whole house is on one zone operating at a single FT. 

In principle the rads in the bedrooms should have been designed for a lower room temp (ie made smaller than would otherwise be the case) but in fact, for historic reasons, they are oversized.  I just turn down the LSVs in the bedrooms, which has the effect of reducing the flow rate, thus increasing the deltaT, thus reducing the average flow temp in those rads only.  Put another way the rads are balanced for the desired differences in room temp, not balanced for equal deltaT.  Does the job!

So once I've set up my water law (which I've more or less done) then I could schedule the hive to be (say) 2C cooler at night than the desired 19C - which would have the effect of limiting the pump.  Is this correct?  Most people say "don't use a hive!"

Not really in the way you describe

The effect at best would be to switch it off altogether until the house cools.  It wont limit it by reducing FT, which is what you really want to do.  However I think Hive may do 'pulse width modulation' on its output, a crude way to control on on/off boilers 'proportionately'.    If it does this (several smart thermostats do) then its a real no-no for heat pumps.   You would need to check this before even considering using Hive this way.  Its possible that this function can be disabled but you would need to check with Hive or someone who is an expert in Hive. 

The Hive guy at InstallerShow definitely told me that they had looked at making Hive properly suitable for heat pumps but had not pursued it because there was no standard interface.  I take that as an admission that currently it isn't!

Samsung has an on/off schedule function inbuilt I believe; use this if you want to switch the heat pump off for a period and leave the Hive doing nothing!

Unfortunately this is where the crudeness of Samsung's controls bites a bit.  That said, although my Vaillant is cleverer in this respect, I still dont bother with a nightime setback, I just have the bedrooms at a slightly lower temp permanently.

PS  There is something here about what Hive does.  The 'time proportional' element is the one which may be problematic, depending on how its actually implemented, which I doubt is publicly documented.

Yes, and probably also Adia although the latter has a much greater scope

Or is Samsung basically saying that it's a manual thing for whenever you change it and then change it back? 

But that would be ridiculous to do every night! 

Exactly that so far as I can tell from the manual and discussions with the Samsung rep at the installer show 2025 (I don't have a Samsung).

Samsung doesn't give an option to have a flow temperature setback or set forward unless you use an external controller.  The only way to achieve setback or set forward using the Samsung controller is to operate it in temperature sensor control mode, which you can do as an overlay to WC.  There is no way to do it if you want to do it on pure WC. 

I challenged the rep at the last installer show about this, he was keen to show me the new flashy full colour UI, which still doesn't do this.  His colleague then admitted that the flashy UI uses the same underlying logic and so hasn't expanded the capabilities to do this very useful thing.  I confess that I told them at this point that their controller was one of the worst on the market of the major manufacturers, which they didn't entirely warm to (but didn't actually attempt to deny).

IMHO this limitation is rubbish.  It's not unique to Samsung though.  If you want to do this sort of thing with Samsung or other heat pumps that have this limitation, you need homely or to write your own script in HA or similar.

We use a programmable room thermostat (first example in @mike-h post above), and simply knock back the desired room temp to 18C overnight, or we can programme it between set hours.

If you don't have an external thermostat, then I think you need to set a 'Weekly' schedule. Are you able to do this, or is 'Weekly' greyed out?

Or is Samsung basically saying that it's a manual thing for whenever you change it and then change it back? 

But that would be ridiculous to do every night! 

I have water law set up (auto), but when I try to schedule, the only options I have are dhw and holiday. Daily and weekly options aren't available...

Richdorm