Anyway, seemed like changing coolant and then bleeding was a good idea.
I'm keen to get all the coolant out of the system and flush it fully before refilling and bleeding. I've followed the method from the maintenance manual (as in the factsheet on here):
- Open top of radiator.
Open bleed pipe bung.
Open reservoir tank cap.
Loosened the drain plug at the bottom of the radiator.
Ran that until it was dry, then pumped water through it with a hose.
Taken off the under cover.
Didn't manage to get the plug on the upper pipe off, so just disconnected the whole pipe and drained that.
Then for good measure disconnected the lower pipe to the radiator and drained that.
Connected everything back up.
Filled the system with water and radiator flush.
Ran until bottom pipe got hot.
Drained again.
Still dirty, so intend to do again.
I've searched through this forum and the instructions for bleeding are very thorough, but I can't find anything as idiot-proof for flushing. And I may well be doing something idiotic and missing something simple, so if you think you can help, please don't spare the details, I'm pretty new to all this, but very happy to get my hands dirty if I have good instructions.