Dave, I use standard square profile Thule roof bars, and a pair of Mont Blanc Barracuda's. They are very secure for mountain bikes, and lock both to the bars and the bike onto the carrier. I've had these Barracuda's for 4 years and they have been great. Previously they were on my Impreza STi and at high speeds and fast cornering (when you forgot there were 2 bikes onboard) they never moved an inch.
I also bought a rigid Halfords rear carrier which is very good, but I prefer the roof carry system.
How far from East Yorkshire are you Dave? You're welcome to try my Barracuda on the OEM bars. I'm 95% certain they fit anyway. They fit most square profile bars.