Thanks to the person who suggested using Vaseline on the side cover posts and grommets. Finally solved the "pushing the grommets through" hassle! I'd used silicon spray without success prior to this. One thing I did though was take the three grommets out first and thoroughly clean the inside of them where they contact the bike to make it as grippy as possible in there. I also now press home the top post first (ie minimising the force required) as this was the grommet that always seemed to push through. Happy days!