Chas, I agree absolutely with John's diagnosis and treatment. If I have the same problem with a small part where a gasket can't be installed, I use zinc chromate paste between the parts where they would otherwise touch. You can probably get this from a pharmacy, or a yacht chandlers, and it's the technique used in the building of aluminium masts on sea-going vessels, when dissimilar metal fittings are attached. If you can't get zinc chromate, dielectric grease or vaseline would be good, although john's gasket is the ideal. Good luck with the polishing. Nick.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."