After some advice from the Rods ‘n’ Sods forum, on Saturday (19th) I removed (wrestled with) the clutch fluid reservoir to inspect the extent of seizure. The photos may indicate the level of corrosion but don’t illustrate the fact that the reservoir was dry upon removal.
I had to cut the fluid pipe to remove it from the engine bay. I then attempted to take off the slave cylinder mounted on the engine block. I got as far as removing the circlip before spraying the unit in WD40 to help the removal of that at a later date.