I have heard that the 10mm is particually prone to letting weeds grow


Presumably you're using weedproof matting under the gravel, so there should be no weed penetration.

Guide to what's available from my local supplier he

http://www.middaggs.co.uk/Forms_and_...ggs-Gravel.pdf

Crushed gravel locks together better when it's compacted, but not so good for vehicle tyres.

Do you want to walk over it, or drive over it?