As Mark pointed out , the quad electrode plug does not necessarily provide a better spark.
Only one electrode fires and any given point of time, and it is usually the one with the smallest gap from the centre electrode. When the firing side electrode then corrodes as result of usage and the gap increases, another side electrode will have the smallest gap and start firing. This way the wear is divided more or less equally between all electrodes, so a four electrode plug will last twice as long than a dual electrode one.
But this is all in theory – for the cost of Ł2 each it is sheer madness to leave the spark plugs in the engine for 80k/8y. I just changed mine at 40k/5y, and considering that in the older days it was a 5k/regap & 10k/replace job, I think that mine was well overdue!
GeffD did say that they were working their way loose even after being torqued-up properly at the factory, mine seemed solid enough though. But a loose plug can cause a misfire, and the cats don’t like that much… so not worth the risk.