Ask the garage what's included in the £900 quote they gave you. As above, if the CSC is leaking, it might have contaminated your clutch as well, you'd need a new clutch, flywheel, CSC and is also advised to get a master cylinder while you do the lot. It will be more expensive than £900, including labour.
Do give you some ideas about parts and prices, have a look here:
https://www.torqen.uk/nissan/350z/powertrain/combo-packs/5880-350z-de-competition-clutch-flywheel-fluid-combo.html
https://www.torqen.uk/nissan/350z/powertrain/ancillaries/338-350z-hr-torqen-tilton-racing-clutch-master-cylinder-trq-350zhr-tmc.html
https://www.torqen.uk/nissan/350z/powertrain/ancillaries/341-350z-hr-z-speed-performance-zsp-heavy-duty-csc-concentric-slave-cylinder-zsp2006.html