I would recommend a good strong boot rack over the roof rack, I used to work in a bike shop and fitted hundreds of the things. Roofracks will cost you about £100 and then £50 per bike for bike mounts, Halfords do their own cheaper versions though I think.
As for bootracks, brands to go for: pendle & hollywood Im familiar with and both are good.
A basic boot rack will take 3 bikes and cost £50 for a good one. If you have a spoiler or anything you can have problems but a standard rack should fit a 172 / 182 no bother.
Make sure you get the straps as tight as possible and tighten them again when the bike is on the rack to avoid any movement. Its a legal requirement to have a lighting board and number plate if you obstruct yours - but you get high mount racks like that pictured by Ally which get around this problem.
I once fitted one to a vectra and told the guy his boot was f_cked he took the car back to Vauxhall and agreed it had been badly assembled and he got a new boot out of it! (nothing to do with the bootrack, just a point to note)