Bit of an old thread but I am about to do the same job.
I've just ordered a genuine thermostat from the dealer but unsure if I need this extra 'gasket' as they didn't seem to know. Is it just the big o ring that sits on the thermostat?
Also, whats the best way to get into it? Is it and alternator out job or take off the manifold? Any help and tips appreciated.
Cheers