These wide arches I bought off eBay for £62 which I think is an absolute bargain, considering I've never seen them before and will probably never see them again. According to the person who sold me them, they were made by somebody in North Bristol who sold a whole kit for the 405, and he's had them (but never fitted them) for about ten years...
I guess that's the problem with owning an older car, the body kits are rarer! I would have love to have seen the whole kit though, what the bumpers looked like, and whether there were rear arches!