How can I replace stat/ divertor valve assembly Ocean Style FF combi boiler?

The honest answer is with great difficulty, with the age of the boiler you will be better off replacing the boiler.
changing the diverter will more than likely result in leaks from other parts of the boiler, you will most likely need to get a seal kit and replace or reseal most of the joints around the diverter as well,

my advice change the boiler

gary.wright53@googlemail.com
March 2008