Ah would that mean replacing the metal master cylinder as well?

Just an update on this. I have dipped my finger in the reservoir a couple of times now, the latest being last week before I went on holiday. There is still plenty of fluid in it. I am now wondering whether it wasn't properly refilled when the clutch was fitted and the level had just dropped a bit as the clutch has worn.

I have, nevertheless, brought a bottle of fluid and syringe with a piece of pipe attached with me just in case!!