So while investigating another problem I noticed that the smoke wasn’t blowing away from the laser as well as it normally does. So I took the drive carriage off and gave the little fan at the rear a good clean. Not much better.
So I unscrewed the air scoop to get at the fan (you only need a little allan key) and the thing was absolutely stuffed with crud underneath. So although the fan blades were clean, the fan grill you can’t see was absolutely clogged up.
So cleaned that all out - including some big lumps of congealed gunk - and now the smoke gets absolutely blown away.