Hmmm more ideas/comments since last night. In summary
Its def M7. I borrowed some calipers and dug out my old engineers ref book and it is not 1/4″UNF
It’s a lease vehicle so drilling and tapping is not an option
It looks as if most of the ‘load’ is taken elsewhere, I ‘think’ these bolts are just there to stop the roofrack sliding round a bit so shear stregth is the most important bit
It’s not a Thule rack so they would not/could not help (although I do have 2 thule racks in the garage from previous cars so could try a blag, but the comment about thier bolts breaking above puts me off a bit…)
Looks like I need High Tensile bolts from a local company that I can ask questions (for such a small sale value as well, I already feel sorry for them)