There is a company at Sevenoaks that does snow chains, and they are the importers for Thule roof bar systems. I cannot remember their name but a web search will give it, search for snow chains. I got a set for a 406 and the ends lock on to the bars, also the bike clamps lock, and the keys were all the same. Shame it dosent fit my cuurent car really.
I've got a Thule towball mounted rack, takes three adult bikes and I can just about squeeze on tiny 14" wheel bike too. It's really sturdy surprisingly enough, even at high speeds (not that I'd go too fast, of course). Only disadvantage is you can't open the boot once its on. Certainly seems a lot safer than those ones that use straps on to the windows!
I believe (don't quote me) that on some roof racks theres a lever thing, so you put the bikes on the rack on the floor, and then lever it up somehow? Not too sure how it works. Although we went for the towball because OH said that it can be difficult getting on ferries and a friend of his lost his bikes, once on the ferry (knocked them off!) and once on overhanging trees!
There have been a few topics on these, try a search?
We've also got a Thule towbar carrier - click on the link above and select "Bike Carriers" to have a look. It's very sturdy, but not sure if you'd be able to open your doors without removing first ?