Open the TTT knob to full open
Drop into a pot of simmering water, add a squirt of dishwashing liquid.
After half an hour scoop it out and scrub with an old toothbrush
Now polish the rest off with Barkeepers Friend
Cleaned up alright, though I found out after I started that I was out of barkeepers friend. I substituted powerful brewery wash (which I think is sodium percarbonate) at 40g/3L.
Pic after the boil and then later after the tooth brushing. Recommendation to anyone else: buy a cheap stockpot (maybe the kmart $15 15L) and dedicate it to the task, rather than having to clean creosote off a good pot (it builds up around the boil line).