I had this problem on my 52 plate...the main cause of the problem is MUD !! The MUD retains moisture and over time it eats away at the metal, get yourself a hose & wash out all the MUD & C... that has collected over the years, i took the arch liners out so I could remove the last scrap of dirt. When it's all dry (next day) get down to Halfords & buy some WAXOIL black underseal and coat all the wheelhouse (especially under the lip of the arch), when that's cured (doesn't really dry) re-fit the liners. Now for the rust.....your going to have to rub down the area and treat with CURERUST & then fill / Paint. Sounds like a mammoth chor but will be worth all the hassle & save you a fortune later on !!