I haven't tried scheduling via the app; I do mine using the main wired controller and just use the app to view.  It does look like some sort of loss of connection that's causing the issues.  Have you checked the Wifi strength in the app?

You can schedule the HW on the wired controller.  There are a few youtube videos like the one below.  If you can run your HW in the cheap periods that will be better, otherwise do it when it's warmer outside.

As for the schedule, I'm assuming you're matching the cosy tariff times.  How are you running the heating?  Is it using fixed flow, weather compensation or room temperature control?   It sounds like you are running it using fixed flow, if so that might not be the best way.

@kev-m thank you very much for this!

I thought it might be WiFi strength, although it was a weaker signal it was stable; I've moved the router closer in any case to strengthen the signal.

I'll do the HW timer as you suggest.

How would I know which mode I'm using for running the heating?

If you look at the front panel of the main controller, WC has a +-0, Fixed will have whatever you set the flow at (e.g. 45) and room temp will have the room temp setting (20 or 21 or whatever).  

You can set the heating mode to change in the app at defined times, although I've never tried. 

BTW my Wifi signal is often 'weak' in the app but it still works

I would concur with Kev M that you are better off setting the target temperature setbacks in the schedule section of the controller, at least until the Melcloud dropout issue is settled. If the temp changes are set via the Melcloud app you are relying on the internet connection to be up when the change is meant to happen for the FTC unit itself to receive an instruction and act on them. The second screenshot you posted I think is just the thermostat switching off a call for heat and the flow and return temps dropping away, as the displayed data looks to be actually correct.

As you are likely operating a fixed flow setting (?) then given the current temperatures of around 0C you would probably be at that 45C output temp anyway. I have a larger house than you, though more modern so better insulated, and I am still getting to that flow temp with a compensation curve. 28-30kWh has been the daily consumption over the past three days for me, so I don't think you are far off what you would be using even changing the mode. However that won't be the case when we hit a milder few days, then a curve should be a more efficient way of running the pump as ideally it is supposed to match the temperature of the output to the heat requirement. The installer should have programmed the curve into the controller to match the heat loss calculations done as part of the installation planning. I presume there is a default curve programmed in anyway so you might have to dig round in the paperwork to find a value for the flow required at -2C and try to work that one out.

Having said all that, you are only heating the house for 12 hours, then for 2.5 in the evening, so you'll have to have to have a higher heat input when running to bring the temperatures back up. 4am is usually the coldest time of the day and not when I would be choosing to start the pump off! But I know that plenty of those who have cheap night-time tariffs do trade in efficiency for the resulting lower energy costs – that is the balancing act of these tariffs!

As for timing the HW, I think you do need to set it on the controller to get the benefits of Eco mode (as shown above there are useful YouTube videos to help walk you through the process, it is not quite obvious when to press which button). How often you need the tank to be heated up will depend on the usage in your house and the size of tank. When you set the time period(s) you'll set on periods and off periods – the hot water cycle is triggered when an 'on' period is entered and will only be triggered again during this period if the tank temp drops by a preset value (default is 10 I think). So you'll want an 'on' period starting in one of the low cosy blocks, or possibly both as I would reckon you want to set an off period to prohibit HW in the peak period.

Thanks both, that's very helpful. From what I've read it seems like ASHPs are less efficient/cost-effective on the really cold days but moreso on the milder days, and that the electricity cost balances out or even saves you money. Does that sound about right?

@edmj if you have solar, the summer months are free for hot water and the shoulder months are quite cheap. Without solar (and batteries) I am not sure the economics would have stacked up for me - but that's a personal thing. Without an ASHP on gas I used 35MW (£3,500) of heat, with a heat pump that same power use was 7MW (£2,300) or 1/5 of the power (based on the cap costs of £0.1kwh for gas and £0.34kwh for elec). With solar and batteries my costs are more like £1400 for the year.

I can say with Solar and batteries my costs are several thousand lower than they would have otherwise been and I have the evidence to back up that claim.