You can buy them from here if you don't mind importing from Germany, delivery was 3 days to UK. You need 3 per side:
http://www.skandix.de/en/spare-parts/body/body-parts/trim-parts/trim-moulding/clip-body-trim/1017092/
Part no. 30678009
Step 1) Snap off the plastic piece that protrudes from the side of the new clip.
Step 2) Slide the new clip onto the stud and push to the end of the slider. The clips are wedge shaped, the tall part of the wedge points outwards and the small pointy edge faces the windscreen.
Step 3) Push down and inwards (you hear a 'click') to secure the trim.
Trim pieces should then be sitting flush against the windscreen and inline with the A-pillar.