Would I essentially be able to do the same for the Heatmeister as a first step, set all to max then control on the grant?  Does that also work for hot water

yes to both, probably.  External controls for ashps (and many modern boilers) are largely superfluous and frequently counterproductive.

The possible problem is if ufh and radiators are set up to run at different temperatures which they might be.  You need to check this and might need to maintain the mixing function of the manifold.  Many heat pumps can control this as well don't know if the Grant can or not.  

Here’s the pictures  

doesnt look to have a normal value to control the heating downstairs only for hot water and upstairs to me but does seem to have a pump for both 

there are 2 separate manifolds for the UFH

• Have you got a buffer/low loss header?
  I dont think so no
• Where is your Grant controller/temperature sensor located?
  Controller is internal, it has an outside temperature on the display
• Does UFH cover the whole house or have you got radiators in part?
  UFH is full downstairs - controlled by the Heatmeister.  Then the nest controls the upstairs radiators and the hot water.

Would I essentially be able to do the same for the Heatmeister as a first step, set all to max then control on the grant?  Does that also work for hot water?

Thanks

But wait an hour or two and someone else will jump in with some Nest insights.

Absolutely.  Dont use it!

Seriously Nest, like other smart thermostats (other than Homeley, Havenwise, Adia), are wholly unsuitable for heat pump control.  You should be able effectively to disable it by turning the target temperature up to max and setting it to be on 24x7, so that its constantly calling for heat.  Then  use your heat pump native controller to adjust WC so you are comfortable.

Once thats done settings can be finessed if necessary (eg by using zone valves/TRVs as limiters in case of solar gain) but until the basics are right then any temperature sensors/thermostats should all be set to max so the heat pump can do its thing without interferance.

• Have you got a buffer/low loss header?
• Where is your Grant controller/temperature sensor located?
• Does UFH cover the whole house or have you got radiators in part?

balance those knobs so the numbers in the clear part all match? currently they look all full/red

Well I can get all my UFH flow monitors to show "all at max flow" simply by having the circulating pump on full!

But that's not efficient of course.
The shorter loops would be returning water which still retains lots of energy.

In your case there's also the issue of the upstairs radiators.
Somehow they need to be balanced with whatever the ground-floor manifold is taking.

Have a look at this article about a flow-regulating valve for radiators.
It's similar to those on your UFH manifold but allows you to see the water speed for each radiator.
If you read to the bottom of the text, you'll find a discount for Forum Members. 😊 

Q. Then how do it make sure the valve (not sure where this is or if there is one for the downstairs system) and the pump are "open", as assume they're also in that heating control box.

You've got manual butterfly valves at the end of the UFH manifold, which don't have to full open. Mine aren't on my upstairs manifold.

But I assume you're referring to a motorised valve and a speed control for the pump.

The only motorised valve I'd expect to find is the 3-port (2-way) unit which shuts off the space-hating circuit when the DHW cylinder calls for heat.
That should continue to operate as normal because the control system is dictated by a temperature sensor on that cylinder.

If you think we need to know more about "the" pump then post a photo.
But do you actually have one?
The entire system might just be using the pump inbuilt to the ASHP.

Q. How do I then set the actual temperature? Assume its controlling the flow temp at source and some trial and error until thats correct?

You should have an Outside Air Temperature (OAT) sensor, which connects back to the ASHP control unit.

And there's a system called Weather Compensation (WC) which allows the installer to select an appropriate WC curve for your house.

Those two mechanisms 'instruct' the heat-pump how to choose the required flow temperature and speed.
Setting up the WC curve is one of most often discussion points on this forum.
Type WC curve into the search box (spyglass at top right of this screen) and you'll find scores of results!

Q. That'd count for the Neo's but then obviously there is the Nest to consider.

That's beyond my knowledge-set I'm afraid.

But wait an hour or two and someone else will jump in with some Nest insights.