Yes Carl M10, checked on my EPC as well
, it's just that on 'partsbase' the description on the right lists the nuts as M8 & M10
(as seen on your capture above).
I noticed it in your post # 6 above
#5 90570845 NUT, HEX., M8, FRONT EXHAUST PIPE TO CATALYST
Edit: Your capture above was for the Astra (I know the parts are same in this case) but if you try doing what you did above on partsbase for the Vectra B, it doesn't list the studs as M10 x 25.