I've just had a bit of a senior moment, but thought it would be worth mentioning...
Cable Free Play: Bear in mind that you can't really measure this by wiggling the twist grip. There is slack in the twist grip mechanism that has no effect on play in the cables.
Being a bit of a knob and not thinking things through properly, I took out the free play at the twist grip with the throttle cable adjusters. This actually takes the slack out of the chain thingy in the twist grip and has pulled all of the free play out of the cables and then some!
Now my carbs are truly screwed!