When desperation evolves into its next phase, remove the plates and see if there's a simple o-ring to be replaced. Keep finger pressure on the plates as you remove the screws then release your finger slowly in case there's a spring waiting to jump out.

Alternatively, is there a Lucas service center (centre?) with a friendly technician? Or one who can be made friendly over a pint? Or any Diesel technician?

This fellow looks thirsty - http://www.lawsondiesel.co.uk/

Sixto
87 300D