Well, after sorting the spark plugs, I ran the engine and had another cough. So this morning I decided to have another go at diagnosing the cause, starting by removing the dash again! After a while I was under the bonnet, waggling wires when suddenly I found the cause. The main loom joins the starter solenoid on the low nearside, and that wire, when moved, resulted in a cough. Because I couldn't look at the dash while waggling this wire, I had to invite MrsW into the garage to watch the alarm/immobliser light as I waggled, and that was it. So I removed and refitted the two cables, and tested - it now seems OK, so I'm confident that this was the cause.
