The engine had a slight miss at idle and it was getting decidedly lumpy. I did some preliminary checks and narrowed the miss down to No 3 cylinder. After a bit of swapping around, it seemed the spark plug and HT lead were not causing the problem. So next thing to check for was valve clearances, maybe one was a bit tight.
I was waiting for my hands to warm up before I went out to the carport. 10 degrees. Felt a lot less.
(For those who're interested: that's a 1976 Triumph TC2500, straight six, twin SU carbs. And yes, that's garden hose for breather pipes, doing a damn fine job too.)
Later...all done. Engine definitely running smoother. Halleluyah.
Having had the rocker cover off, and done my best with the broken cork gasket using silicon for joint repair (ugh), think I have a bit of an oil leak onto the manifold. I'll need to redo the gasket. Job for next weekend, like I need any more on the list...