Investigated the spongy clutch this evening. Hopefully the problem is this ill-fitting lid, or perhaps the fluid has just run out over seven or eight years of standing still. Of course, there’s also the possibility that it’s leaking like a Reading Festival urinal.

Once I removed the lid - with mole grips - I found enough crystals to keep the Aztec Zone in business for several seasons but very little clutch fluid. This explains a lot! So hopefully, a quick beeding will sort it out. Might also need a new master cylinder, if I can’t find a new lid.