Massive thanks to @Blyatsu for this!
Ever since I’d removed the engine to replace the clutch, the Sirion had run cold. I assumed that all the wiggling about and drying out had caused the thermostat to jam open, but chanced upon this thread which gave me an idea
In my haste to reassemble everything, I had plumbed the top and bottom radiator hoses in backwards on my car too.
I Undid them and re-routed the smaller diameter hose off the t-piece (which was all muddled up with the main loom and clutch cable) and the refilled the system with new coolant. I didn’t have a chance to take it for a drive last night but will go out later this afternoon and see if it gets up to temp properly now.
It might still be the thermostat, but I’m hopeful that its just that I’m an idiot and don’t take enough pictures of dismantling things to refer back to when I’m putting it back together again.
I do have a £20 Gates thermostat here that I can fit if needed, but hopefully its sorted.