Cheers guys phew! I've not fucked the piston by screwing it in!

Also another rear caliper question. When the handbrake spring came off I took the bolt that holds the spring loaded against the hook and the whole hook raised up by about a 1.5cm out of the caliper I hammered it back down and bolt the bolt back on.. is it normal for this to pop out? I guess the bolt usually holds this down.

I really hope the above makes sence (thats why I didn't mention it before)