Not sure on the cost but it will definitely last if all the rust is removed, welded properly, and painted. Does the place you got it done at first not offer some sort of guarantee?
i dont know if rear arches are available mate but my bodyshop friend said it would cost around £250 a side to weld and paint a new arch in if they were available mateSouthpaw: yes 12 month guarantee which has expired
Speed: any ideas how much it would cost to weld in 2 new arches & re-paint? Are the arches readily available to a bodyshop?