Grrr, not so simple. I drove it to work today (so I can take it to a garage at lunchtime for something unrelated), and it got about three miles before dying in a very familiar fashion.
I waited five minutes then tried to restart it again: it caught, very very intermittently, but definitely wasn't for starting.
I tried flicking the points while holding the central HT lead near something metal: I got a good strong spark.
I tried turning it over (by shorting out the solenoid with a screwdriver) while holding the HT lead to plug 1 near something metal: not only did I get a good strong spark (right up my thumb) but it happily started on 3 cylinders.
And then I drove it the rest of the way to work on 3 cylinders.
The brand new coil felt quite hot, but no more so than the top of the rocker cover where it is mounted.
My next plan is to bypass the spark amplifier, and drive around like that for a while to see whether the problem recurs. And I'm going to buy replacement HT leads, dizzy cap and rotor arm, but not fit them until the problem occurs again, so I'll know what change fixed it. (Always assuming that *something* fixes it!)
Other suggestions gratefully received!