Yes, the problem is this spigot to the pipe shown by the green arrow. Someone has replaced the spigot but with a slightly smaller one leading to a long term slight leak which wasn't traced, it was always assumed to be the pipe of death. As they say, assume nothing...
They lined the pipe with something that eventually hardened / cracked which caused a sudden coolant loss. And that's why I initially thought it was the pipe that had hardened.
You've answered my question regarding the entire assembly, as shown in your photo as below in that this isn't normally disassembled and the circled connector isn't available separately, so I don't see any point in separating it. The assembly seems to be available new, £90 (I will find out if it's actually the case) and should be second hand, for now.