I had noticed I was slowly losing coolant, and had attributed it to slight leakage at the head gasket but after running the engine up to very warm, the heater return line showed up something interesting. It began to bubble.
Decided to replace this piece of shaped molded pipe. Typically, the side easiest to get to came off, but the difficult to reach part had to be removed forcefully.
Coolant still looks good though (it started out life yellow).
I think it’s an interesting manufacturing flaw, there are 2 similar pinholes along the length of the pipe.
Managed to find a roughly equivalent piece at the store.
Got it all fitted up and bled.
No more leaks!
…or are there? Radiator was a bit damp. Thankfully that is from where I’d opened the bleed screw on top of the radiator and it had percolated down the fins inside and come to the surface there.