You get a better and more substantial assembly than with the OE on the ZS180. Like those fitted to the 620ti and 800s. These later ZS OE hydraulic clutches are IIRC, sealed units which cannot be repaired, only replaced.
When the one on my ZS starts to play up, I shall replace it with that kit offer as it's far superior. If, or when any individual parts wear out, they can be replaced or repaired separately.
When the clutch slave cylinder on my 620ti had worn seals and behaved as you describe, I renewed them with a seal kit ~ cost 6 quid from MG-Rover spares dept ~ now XPart. Good as new.
Search here on .org for more details of the kit..... much less than 200 quid IIRC.. which is not always the case as I can rarely line up my remaining two grey cells only occasionally...

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