Okay, first off, a bit of advice, DO NOT EAT A TOASTED CHEESE SANDWICH NEAR YOUR MACBOOK WHILST IT IS OPEN!
I simply cannot stress this point enough, if I had paid attention to this it would have made my life much less complicated.
Okay, so you have either a bit of hair, or a stringy bit of cheese under a keycap (in my case the 'B') the way to remove it is quite simple.
Mini swiss army knife (I used this one)
Pin (not needle, you need the enamel head)
Using the screwdriver blade, slide it under the keycap and prise it up very gently, not enough to remove it but enough to see the small scissor mechanism.
Use the tweezers or the toothpick to gently press in the scissor legs on the LEFT hand side of the key (on the edge facing you), it will move slightly and the key will pop up away from you, lift it up and slide it to the right to remove it from the clip.
Now clean out all the crap under the key, I had a bit of cheese (see above) and a hair.
Reinserting the key is a different story, its a hell of a lot more difficult! It took me a few goes at it before I had a brainwave. What you do is insert the pin between the legs of the scissor mechanism to prevent it closing. The head of the pin prevents it falling through so just let it sit whatever way it suits you.
Line up the right hand leg/clip thing and slot it in to the left, then just drop the key in place and then press on it gently, making sure that it is lined up. It should click into place and you are now done!