Is the little pipe from the top right corner of the radiator to the header tank clear of obstructions? It should have a steady flow when the engine is running. My D2 Td5 hose stays hard, too, I don't think that's an issue as the system runs at over 1 atmosphere pressure, so if it is staying firm, I'd say it's still well sealed and in working order.
I'd pull the top hose off and reclamp it with a smear of rubber grease on the inside of the hose for starters, and then run it and see.