Hmmm... I always check the leads to the plugs, and there is often at least one where the braiding is worn through and the white inner is showing. I was lead to believe this was normal, so next time I'll certainley be giving them a gentle tug to see what happens. Thanks for the 'heads up' Thumpango. I suppose having two plugs in each cylinder means the a/c isn't exactly going to drop out of the sky if a lead DOES work loose in flight, but still worth avoiding.
Incidently, I also fly the Robin 2160 where it isn't possible to check the condition of anything like this as the cowling is fixed in place. Surely these things are important (as demonstrated above), in which case there should be a way of checking.
[Edit: Actually, thinking about this in hindsight, perhaps it's everybody tugging at the leads 7 or 8 times a day that causes the problem in the first place, who knows?]