OK then you'll need to remove and trim the plastic arch trim.
Then you will need to roll the lip on the arches. You can do this by heating up the area gently with a paint stripper gun and then you can carefully roll the lip flat with a baseball bat or broom handle levered against the lip and the wheel. Go carefull do it a little bit at a time and keep the paint warm to stop it cracking.
Or you can buy a proper arch rolling tool for over £100, or take it to a bodyshop, or I seem to remember there is a firm in Exeter who hire out the tool, I can look out their details if its close enough for you to collect it.
Also worth looking on ebay I think there was a few firms hiring them out on there.
There was an interesting vid on youtube about arch rolling - some suicidal Jap did it with a thick metel pipe trapped between the wheel and arch and then drove his car forward - nutter.
Paul