Mostly it would fire about 3 times and then nothing. The computer apparently thought it was firing but still detected too much oxygen in the exhaust, so injected lots more fuel, flooding the engine. When the RAC guy pulled the plugs out they were visibly wet. Turning the engine over with the starter and foot flat to the floor would eventaully get a few more fires, then running on one, then two, maybe 3, and suddenly back to normal, unless the battery went flat first. On the last few occasions, it ran about half a kilometre and then died again. But more of the same got it going again, then OK for the rest of the day.
Water is a lousy conductor unless it contains ions - dissolved salts. Oxides, like the build up on my inside terminals, are even worse, effectively insulators. But zinc and copper oxides are also hygroscopic, so they draw moisture out of the atmosphere. Being warm when you stop the engine, the air under the bonnet holds a lot of moisture. As it cools down, it condenses out. More so when the humidity is high, like on a wet night.
Putting a towel over the engine at night absorbed a lot of moisture from the air under the bonnet - it was usually a bit soggy in the mornings - enough to stop the problem, temporarily.
My reason for writing the follow-ups is that other people obviously get the same problem, but rather than rush off to get a new dizzy cap, I think it is worth at least looking at the old one. You have to take it off to replace it anyway so why not? Make sure you undo all 3 screws, and check that the inside terminals are shiny clean on the sparking surface. The spark can jump around a bit of build-up, but eventually it gets to be just too much.
