Tech3 kit. Fixed my "squeaky / juddery" piston, however I think a proper bleed would have also done this had I tried it first. Kit contains everything needed, but youyube is your friend for instructions. Piston seals are a nightmare to get off and back on.

Tools needed - Very small external circlips - I used Knipex 49 21 A01 (3-10mm) as the smallest Park ones (RP-3) are not delicate enough. The torx inside the lever is also a security style head.