Diagnosis so far
I pulled the spark plugs one by one and the engine continued to run very poorly, with no change noted.
Checked the OVP relay (the 1981 still uses an internal fuse - this migrated to an external fuse on 1982 and later) and both the diode and fuse in the relay checked out OK. It's in a red metal can cube above the fuses in the passenger footwell and has a pin-out diagram stamped on top - very helpful.
Checked the vaccum hose routing and noted that the hose (white/blue/purple) from the ignition advance/retard unit (on the ignition distributor) to the air pump (pumps air into a/r unit) was fractured. This runs across the engine and must be subjected to a lot of heat - it is so brittle that parts simply break off if you try to flex it.
I will replace this weekend - could that be the source of my problem?