I took mine down to Discount Trucks and used the train to get back as they do really know what there doing! Not to say there isn't adequate shops nearer but mine were done 6-7 years ago now but replacement arches and sills are now available specifically for the Bongo from BongoSpares.
Best to get a current quote, prices are changing upwards all the time, allow for at least £200+ per arch as a minimum and make certain that they will cut out all the old, weld in replacements and re-paint.
See here for the process http://www.g8dhe.net/bongo_images/Paul_ ... he_arches/