Pop the dust seal, apply WD40, go again.
Not wanting to state the obvious but some people fail to get this right...
... the rear pads have a locating boss on the back of it which needs to sit in the grove of the piston. If it doesnt the pad isn't sat correctly and so sticks out further. Be sure you get this bit right.
Mick
You have pulled the brake pad carrier away from the caliper haven't you?
I made this mistake once you need to push the caliper and pull the carrier away from each other
no that is a bodge
the correct method is to smack fook out of it with a hammer, won't fix it but it'll be fun
Brake parts.com, I forget the actual site.
Google brake parts, it's a green and white themed site, but is flash based so can't do it on an iOS device.
About £25 last time I checked, it's a very common caliper