A: Definitely replace the whole new cartridge, sometimes it can be difficult to remove the old one and it has to be broken out in pieces but it must be completely removed.
When fitting the new one it is a good idea to a thin line of superglue to hold the seals in place in the cartridge and apply silicon grease to the seals then ease the cartridge in, if any real resistance is felt pull it back out, check for any obstruction or movement of the seals and try again.
The cartridge can be ordered online by clicking on the link below.

http://www.showerdoc.com/shower-spares/triton/mixer-showers/TRITO-83304950-LP-valve-cartridge-(built-in-valve)
Answered 5 days ago | Alastair

A: The 0.5mm dimension is well within tolerance and will not cause the leak, this will be caused by the seals not fitting properly and the solution is to remove cartridge and check then refit.
Answered 3 days ago | Alastair