To answer my own question, in case somebody else wants to try this cheap approach:
Overall the cost ended up higher of course, having ignored all those little extras and the high price of copper pipes. All in it was £1400, without dhw support, just heating. Also my dad had been very generous in the number of radiators and used 18mm pipes throughout, so I didn’t change anything in the house, just connected the heatpump, plus expansion vessel and magnetic filter and y strainer, plus one anti freeze valve and removed all the thermostats. The house has only one kitchen/living room, plus two bedrooms, so as long as the living room is the right temperature I’m happy.
The heatpump itself seems fine, a little small maybe, which is going to lead to less then ideal efficiency. In the £551 that I payed for the refurbished heatpump, you get the Heat Pump and a tiny controller. The controller doesn’t include an indoor thermometer, so weather compensation or flow temperature control are the only control strategies.
Where it all fails is in the software, the WiFi, online app seems to be abandoned Chinese software, or I need to call the suppler appliancesdirect to create me an account. i havent bothered yet to do that, I simply added a Shelly switch so I can switch it on and off remotely and added an automation to home assistant to create some frost protection.
To set the heat curve you have to set four outside temp/flow temperature pairs. You need the manual when setting anything, as all the settings on the controller only have a code as description on the controller and the manual doesn’t quite describe all the terms either. On the other hand heatpump plus controller cost less than the Vaillant controllers
The controller does report various measurements, but no flow rate, cop or electrical consumption. and no history. All it has is current flow and return temperature.
From the Shelly switch I do have the electrical consumption and it varies between 700W and 2000W, depending on outside temp. It looks like either I got the heatloss consumption wrong or the heatpump not overly efficient.
So overall it’s another case of you get what you pay for and this is all I wanted to pay for this as the house is rarely used I’m happy enough I’d be happy if the app worked and I could remote control it better. I might try an mbus adapter and home assistant at a later date or really splash out and install a oem kit, but that might blow the budget
