[Martin Y]

The usual cause of a clutch that won't disengage is low fluid caused by a leak in the clutch slave. Sometimes hard to spot because fluid that's leaked out fills up the rubber boot around the actuator. So it's a bit of a mystery that you didn't find the fluid level is low. (Sorry even to ask but just be sure - you did check the clutch fluid reservoir and not the brake fluid, right?)

I guess going from here I would start by replacing the clutch fluid.