Reading Abdullah's message regarding the seizure of the viscous coupling has prompted me to post a message on prevention of this occuring in the first place.

Working on the theory of prevention is better than cure; this problem is easily prevented.
Using a HIGH TEMPERATURE spray grease, available from engineering suppliers; spray the coupling around the spring and pin, do this on an annual basis, or when replacing the drive belt.

Do not use ordinary grease, as the viscous coupling can generate a lot of heat when it is in operation, high temperature spray grease is available for around £5.