I'm not sure if this is the right place to ask but I'm thinking of using one of the roof bar mounting points at the back to attach a CB antenna mount. What size is the thread and what torque can safely be used?

You may have to 'tap out' the thread before fitting?
I ended up having to do this when fitting my roof rails, as I snapped a bolt in situ because it was overtight - big job at LR to then get it out.
Bolts used for the rails at the rear are M6 x 38 [but your length will depend on what thickness the base of the antenna fixing is]
They're torqued to 12Nm FL2 now gone and have a couple of items left for sale:-

Brodit active iPhone holder + Brodit dash clip,
Bluetooth Music Streaming Module.

Thanks for that. Even better if I do need to run a tap through them, I'll have a nice clean thread.