Thanks for everyone's help. It does appear to be a from a Transit. It has been sourced and is currently on its way to me (it is in "transit").
We bought the car new in the UK where we had it for a year. It was then shipped to New Zealand where the father in law borrowed it for 3 years. We have now had it back for 9 months. During all the time that we have owned it, it has been serviced as per the manual. It hasn't been driven on salty roads.
I am not sure why only one side has rusted out and the other is ok. The brake fluid is clear. I can only surmised that it was faulty, unless someone has a better idea.
