That's because you haven't 'indexed' them Craig. You have to make sure that the gap on each plug is aligned to the exhaust valve...or maybe it's the inlet valve...or possibly to the East. I can't remember :)

Haha, indeed :) Although I recently read an article about race-preparing a 250cc Paton (a rare Italian GP race bike from the 60s, a twin-cylinder, twin-cam 4-stroke). The mechanics added or subtracted spark plug washers to get the electrodes' 'reach' into the combustion chamber optimised. I suppose at racing level, every 0.25bhp helps ....

Iridium plugs are really for a) high-output turbo petrol engines, as combustion chamber pressure increases the voltage needed to jump the spark plug gap, so having a very narrow electrode that doesn't erode is desirable for a reliable spark

I thought turbocharged petrol engines had lower compression ratios than NA engines to avoid preignition. The turbo will make the inlet mixture hotter, so it's more susceptable to pinking.

Lower compression ratio, yes as calculated but the actual pressure is higher because of the turbocharging.