Well, it's taken a while, but I've finally got it all working! Blown away by how well it works tbh, can get the bedroom radiator up to 60°C in about half an hour, or with the saloon radiators on as well, up to 48°C in around an hour.
Was definitely a struggle to get it all leak-free, I've learnt a lot about how to use PTFE tape and Fernox effectively, and am now competent at draining, refilling and bleeding my skin tank after having done it three times over the last couple of weeks 😂
Everything could definitely be neater but overall I'm still quite proud of the result. I gave up on the non-return valve in the end as I couldn't get it leak-free without screwing in the push fit connectors so far that they got in the way of the flap. Used a lever ball valve instead which in retrospect will also be useful in the summer to quickly shut off all the radiators and use the eberspacher purely to heat the calorifier.
I've got the big red gate valve completely closed at the moment and have been monitoring the engine temperature with the infra red thermometer, which seems to be normal (around 80°C at the hottest point around the exhaust manifold). Will have to see what happens when we go for a cruise, as we've not tested it with the engine in gear yet. I'd really like a proper temperature gauge though, our engine only has a warning light. Has anyone got any suggestions for what, and where we could install one?
Anyway, some pics below, feel free to point out everything I've done badly! 😂