martin young

What do you mean? Blows the plug? Is it burnt? Or sooty? Wet? What colour is the plug electrode when you pull the plug out?

Does it have a headlight switch? If not there is most likely a relay that powers up the headlamp when the engine reaches certain revs...like 800 or so. this si to stop battery drain prior to starting the motor.

Breather on top of the engine. Well........posting a pic would be most useful. Where does this breather exit? Under the engine? Could it be the airbox drain hose? Has the air filter been over oiled?

As said - low oil level. R6's drink it so keep an eye on it.

6' 5" ? On a 400? You'll die of cramp. Get something more suited to your size......like anything but a 400! If you want something that's not too fast how about an MT-03? Or an XTZ660? Or a TDM850? Fazer600?

Sticking thermostat? Siezed or broken water pump? How do you know it's overheating?

Depends how loose they are. The o-rings will swell a little when they are immersed in fuel.