Cool cheers guys, I was mulling over getting the Yamaha ones if the caps were the cause of my misfire (but the cheap ones to see if they cure it then get the proper jobbies), but I'll stick with the NGKs as they sound good enough.
I'll also dob a bit of grease on all four plugs to be safe too.
You say you have a misfire, you sure you haven't got moisture inside the tank if the bike sits in the rain a lot, and you use it all year round?
I've just cured months of starting woes by putting 99% Isopropyl Alcohol in my tank and it's running absolutely fine now, and starting up first button with no problems at all, no matter what the weather.
I put 400ml in a full tank of fuel, then 300ml in the next 2 full tanks and it's worked wonders. The alcohol binds with any moisture and burns it off as fuel. Otherwise moisture just settles lower than fuel as it doesn't mix with it, hence my starting problems as it was sitting in the carbs after settling over night, or all day while I was working.
5 litres is cheap enough to buy on Ebay, and it won't affect the running of your bike other than the positive effects.
It's kind of standard to spray WD40 around the plug area as well to repel unwanted water.