I did have the thought that it could be a fake, but it came from John Lewis as far as I know.
It was a birthday present from my mum, but I was renovating the house at the time, so it was stored unopened in a spare room in the box for about a year. This was why I didn't originally complain, because the receipt would have shown it was a year old, even though I hadn't used it.
The white stuff is ash. I'm pretty sure it's mild steel, but I can confirm that. Ironically I'm a metallurgist working predominantly with high temperature metals. Ash is well known to be very aggressive in terms of hot corrosion (think volcanic ash cloud) and a lot of historic knowledge on this stuff comes from places like the coal board and power station operators.
To me it looks like a textbook case of it being made from an unsuitable material and the design having been updated to a suitable material (stainless steel). But, I am surprised that there aren't lots of people having the same moan on the internet, if Weber had of made a lot of ash catchers to this specification.