What you want to do though is get an additional luggage strap ,the buckle ones with a hook on each end are good.With the bootlid up pass it around the hinges and onto the roof .When you close the boot lid and put on the bike rack you will have an extra means of securing it to the car ,not just the hooky clip things.I also slip on a length of inner tube on this to prevent chafing.A belt and braces approach ,you might say.
oh ,also ,the hooks that go under the bootlid at the bottom.Just hold them in place with a bit of gaffer tape.on the inside,you wont see it.These can also unhook. Not disastrous ,but the rack becomes unstable as the weight 'settles'.
I do all this because i once saw a bike rack hooky strap thing snap on the M67. You will be surprised how high a mountainbike will bounce at 70 mph.
I also do this because my bike is worth more than my car . Hmmm