whih arches did you get? do you have a picture? If you got the geniuine archesfrom another VTR/S then its good news. If you didn't then still shouldn't be too bad.
What do you mean by flat back? Most bumpers are designed to fit the VTR/S models so the bumpers are slightly wider where they meet the arches, which should still be ok with your arches.
Thats pretty much the only difference. One option is to get the entire VTR/S kit from a breakers and get it fitted to your base model. This will include 4 arches, bumpers, sideskirts and door trim and panels.
Or there are kits designed for the 1.1i base models.
These are some pics of the cliche kit: