It's off.
In the end, it only took about 10 seconds with the Clarke mains electric airgun to shift it. All very undramatic, it just came off. So electric trumps air in this case. Proof shown below.
With the (very heavy) triple groove crank pulley off, I was able to turn my attention to other stuff. The first thing was to see if the water pump pulley off the seized carb engine would fit. It wouldn't - there was a sensor bolted to a metal bracket with a pointer that was in the way. I guessed that it was something along the lines of a crank position sensor for the injection engine, and decided to take it off. With it off the carb water pump pulley fitted perfectly and lined up with the carb crank pulley. The sensor is shown in the second (also visible in the previous photo). Can anyone confirm if my diagnosis was correct ot not?
I then lined up the various pulleys and marked them with Tippex ready to change the belt. When the crank pulley was lined up exactly with its alignment mark, the aux pulley looked about half a tooth off. Is this an acceptable level of variance, or is something amiss? (And before anyone mentions it I will very definitely be changing the idler bearing...)
And lastly, with the crank pulley lined up correctly, one of the camwheels looked a bit off, as well. Is cambelt changing an exact science, or is there a bit of leeway?
But anyway, at least I feel like I've made some real progress. And while I had electric power available I got the multitool sander out and gave the flywheel drive face a good clean up, followed up with plenty of brake cleaner and scrubbing.