Thanks to everyone for their prompt replies.
The question of MOT is an interesting one. On the last one in June 14, the corrosion on the dust covers was listed as an advisory. In addition, my nephew owns the MOT test centre and he's told me it's a failure. However Markrat says that he's taken them off and his car has passed since. I know MOT test Centres are far more strict now but perhaps it's one of those things that can get missed.
I could do with hearing from someone who's actually sourced some replacements?