Peter,
Very nice. this has prompted me to add some of mine.
I don't know where you stand on the 'to clean or not to clean' debate, but in this case, where the gilt seems to have been lost, the metal work will polish up really well without damage to the helmet. I usually clean a helmet when I buy it and then leave it alone. I take off all the fittings where possible and clean the skull with car upholstery cleaner . Once it's dry I go over it with a clothes brush. This process both cleans the fabric and raises the nap. Where the gilt is still present I just wash the metalwork in warm soapy water. Where the metal work is back to the base metal having once been gilt , I clean lightly with brass polish. I also polish white metal fittings. If you don't like using anything abrasive Worcester Sauce can also be applied. Leave if for a few minutes and then wash off.
I don't recommend Coca Cola- I used it once on a filthy cap badge as an experiment. It certainly cleaned the badge almost instantaneously, but it came out almost too shiny to be true. Can only imagine what it does to tooth enamel! Before all you guys in the US take umbrage at this slur on your national drink I'll hastily point out that my great-uncle, who made soft drinks, had the the Coca Cola franchise for Northern Ireland during the war and consequently helped the war effort by making many GI's very happy! Sorry- a bit off topic!
Patrick