Retro fitting roof rails

Hi - just wondered if anyone has retro fitted Discovery 1 roof rails before, and can advise what size holes I need to drill in my roof?
I thought this would be a straightforward task (!!) - the mounting points are located internally at the correct centres, so I thought I would just drill upwards with a small pilot drill, then back down with a 10mm bit to take the M8 fixings - bolt them up and job done.
But - when I sit the bars on the roof, the heel of the captive housing on the roof bar itself sits awkwardly compared to the profile of the roof.
So I'm wondering if I actually need to drill a 25mm hole so that the captive housing itself pulls through the roof skin.

I attach a couple of photos, showing the internal mounting point and the roof rail sat on the roof demonstrating the conflicting shapes.

Grateful for any advice!
Cheers,

Pete

Yep drill larger hole so pulls through outer skin
Use plenty sealant aswell stop leaks pretty straightforward job

i used a hole saw to do mine

ok thanks guys, small pilot hole followed by 25mm holesaw then!
Cheers,
Pete