okay - so tried to see how I could remove the trim as when washing the car at the weekend a piece of paint actually flaked off leaving a horrible white powdery mess behind

It turns out that you do have to remove the top section of the door trim as well as the door card by the looks of things as the fixing sits about 2 inches below the door sill. Frankly the thought of pulling the door to pieces without much of a clue as to what I'm doing put that idea to rest.......so plan B - repair in situ

This actually turned out to be pretty straight forward - in fact the more coroded the trim the better as the gloss finish is sprayed on to a sort of rubbery/plastic coating a couple of milimeters thick on top of the aluminium - what seems to happen is that water gets between this and the aluminium and 'blows' causing the white powdery effect and 'bubbling' behind the paint. Where its coroded the paint and rubbery stuff can be just pinged off with a flat blade leaving the aluminium exposed below.

Mine was only gone in the corner so the rest had to be sanded down to the bear metal (this rubbery finish won't let you get it smooth and feathered in to the aluminium no matter how hard you try as it just has an 'edge' to it) so I sanded thru grades 240 all the way down to 1200, primed, then painted (3 coats) and then laquered (another 3 coats)

Now, with it in situ you can only get to the visible part above the rubber seal on the window, but if your patient and careful you can mask it up and get the paint just below the door seal so cannot see anything even on close inspection.

So, 5 hours start to finish in elapsed time - in total only about 2 hours work (rest was waiting for each coat to dry) and bingo - nice shiny trim again