RMcB

I just couldn't manage to separate the plastic central part (with the lock) from the bezel, so just masked it off and got stuck-in with (three grades of) steel wool.
The results were so much better than I expected, even although I hate anything remotely bling, the level of "shiny" was spot on. No need for paint - the brushed aluminium look is great.
Sorry if this series of posts is now in the wrong place, and has turned into a blog rather than a "how to", but I thought that others might like to know that good results are very possible without separating the two outer parts of the fuel filler cap.