Thanks for the replies. Just been looking further at it. Mostly bad at the front where the water drains off the roof and down. Actually looks worst in the photo. The front end is worst, and the sides. The back is completely clear. Just curious as the front and side look as if they have been coated. The metal looks shiny underneath the black coating, almost looks stainless and not galvanised. Another job to do on top of my other list of DIY :). Thought I'd escaped such buying a newer model.
Cheers
Kenny