Neil, I suspect my mid 80`s +8 has similar alloy bumpers to your Mog, though I have never polished them.. I do not mind my Mog acquiring a degree of patina.. (-: I have over the years polished a lot of alloy, even the innards of inlet tracts and much else. in years past usually ending up using a polish called Solvol Autosol which was a slightly abrasive paste, however if as you suggest your bumper has a coating that could cause you to remove the coating and end up with a very highly polished bumper that will dull in jig time.... My polishing days are long gone, and the products I used may no longer be available, Brasso was another, and there was a tin of something similar to cotton wool that was soaked in some sort of polish which was only just abrasive enough to remove insects and provide a shine....Duraglit..? There are very similar kitchen cleaners with varying degree of abrasiveness, that might be worth a try...the very mildest of which may be Sainsbury`s all purpose cloths...
I would use the rear side of the bumper to first try out any polishing materials, to save creating a problem for yourself.
If there is one chap who has great experience in polishing Trad Morgans, it is Mr GoMoG ( Lorne) who despite many thousands of miles covered, every aspect of his Morgan is polished like a new pin, though from memory he has chrome bumpers... naturally... (-:
