However, they really do not seem to feature in any Best Buys, etc. and I wondered what your experience has been overall. I am up in the top right hand side of Hampshire!

Thanks. Kerry

suggests that one might only be for Midea air-con units. If you have any python system around, it doesn't look too difficult to "pip3 install msmart" and then "midea-discover" to see if it can see your heat pump.

Yes, all the stuff I have found is for Midea AC units, but I reason they might be very similar. 

I also tried using midea-discover (after pip install msmart, msart is obvs a python library/module, but midea-discover is a CLI program) and it does appear to discover my heat pump (gets things like 'id' and various token and keys) but it also says the unit is unsupported, because it isn't an AC unit... There's a fetched code that indicates what kind of unit (AC, ASHP etc) it is (looks like a hex number) and if it doesn't match a supported type, the return message in so many words is nice try but no cigar. The Midea handshaking protocols are fiendishly complicated, more suited to banking than domestic heating.  

Don't worry if that doesn't make sense yet: hopefully it will if/when you need it.

It doesn't (yet)! But I keep on seeing the same jargon. I expect in due course it will all become clearer.

Elsewhere, on google, there are some suggestions that home automation and monitoring software like Domoticz can use these protocols. Could we be missing a simple trick here, just wire up a wired or wifi connection to the controller (H1 and/or H2 connections perhaps?), and have Domoticz read and log the data?

I doubt they're on Google and you don't say what you found, but https://www.domoticz.com/forum/viewtopic.php?p=252020 suggests that one might only be for Midea air-con units. If you have any python system around, it doesn't look too difficult to "pip3 install msmart" and then "midea-discover" to see if it can see your heat pump.

I'm not familiar with modbus but I suspect wiring in a modbus adapter is an alternative approach, similar to wiring a cn105 adapter onto my Ecodan (except maybe other devices can do that without losing their cloud connection, which we use for RHI MMSP reporting).

Domoticz can read and log sensor data, but I think it only keeps detailed logs for a day (but summaries like max/min/mean for maybe a year?), so for deep analysis, openEnergyMonitor's emoncms is useful. It may be feedable directly from whatever connection you cook up, or Domoticz can be configured to pass things on over MQTT on a domoticz/out topic. Don't worry if that doesn't make sense yet: hopefully it will if/when you need it.

A few pages earlier (p41 in my copy) it gives a very basic schematic for the wiring which shows A B X Y E and H1 H2 connections.

Elsewhere, on google, there are some suggestions that home automation and monitoring software like Domoticz can use these protocols. Could we be missing a simple trick here, just wire up a wired or wifi connection to the controller (H1 and/or H2 connections perhaps?), and have Domoticz read and log the data? Then we would have access to continuous real time and historical data, ambient, LWT/RWT and flow rate, power input/output, COP (calculated)  etc. We could also probably see defrost cycles (LWT/RWT go the wrong way round), how long they take, how long recovery takes, power used during defrost etc. I also have the external ASHP energy use meter which has a blinking led and an optical sensor can use these to derive kWh used (>> accurate than the Midea figures, because it is all energy used, not just compressor energy used).

@mjr - you may have something to add here?        

What I've read about Home Assistant fits exactly with your description, lots of folks chain sawing through their home automation, and probably taking their legs off while they are at it! I'll have a look at domoticz, it sounds a better bet. Most of the Midea stuff on GitHub seems to be about air conditioners, and adapting it to heat pumps isn't easy because Midea use absurdly complicated security handshaking more suited to espionage than domestic heating/cooling. Then again, maybe they are in the espionage business, hence the security... I did appear to be able to connect to the Midea server, I got back on my PC the date that I first registered the Midea app, and my PC doesn't know that because I registered the app from my temporary mobile, unless the date came from the Midea controller, but I don't think so, but I couldn't get beyond that because the handshaking was too gomblicaded. I was using various python libraries, most of them from GitHub, there's plenty of them, see here. Most of the ones I tried are at the top of the list.    

a defrost cycle the pump in vampire mode, sucking heat out of the building!

I think it might not be because it should be using the heat currently in the liquid on the heat pump side of your LLH and should be over before it cools the water returning from the heating loop much. This is one of the (few?) benefits of your plumbing. It's just heat that never reaches your space heating.

By the way, if yours is anything like ours, it's a pretty epic show if you see the pump doing it: clouds of water vapour spewing out across the back yard, like a supersized gas boiler exhaust.

I will look into the available home assistant options. Any pointers towards some simpler, more robust systems? or where a good place to start is? To a beginner, the area looks like a field full of rabbit holes of unlimited depth.

Yeah, I can understand being cautious. I got onto the slippery slope by using OpenEnergyMonitor's emonSD card on a Raspberry Pi that I had lying around, because Mitsubishi's historical data recording is a bit of a pain in the bum to access (the dates reset every time, for example). I tried at least four home automation systems, including the popular "Home Assistant" before settling on domoticz because it is fairly simply but offers just enough expandability to cope with everything I've wanted to do. Home Assistant seemed more like the mythical Swiss Army Chainsaw: it can do everything, but it takes effort to learn it and use it properly, and if you get it wrong, you don't just shoot yourself in the foot, but cut your leg clean off!

I mention it only because it's one of the few ways I can think of overcoming the limitations in closed manufacturer controllers.