Steve, I took this path four/five years ago. Your AGS was finished in OLD Hammershite and it was probably baked on. The real McCoy. Ain't around no more.
Wickes, believe it or not, do a reasonable facsimile in terms of colour, but it is total p*ss in terms of durability. I repainted my AGS using cellulose thinners in a cheapo spray-gun, and it took three 'go's' to get a smooth hammered look. It helped to cure the paint that I did it in July, but unless you know a car body shop friend, don't eben think about doing it in present humidity and temperature.
Subsequent careless dropping of timber on the paint have chipped it, particularly on cormers or edges, but I had taken care to rust-proof and undercoat the beast, so, so far, no bubbling rust and lifting paint.
Sam