My front arches went on fine mate, no mods involved for the front.
The rear you have either got to cut the pretruding part of the 1.1 arch or i suppose you could try and moddify the arch your putting on by fibreglass or something. Not sure how it would look though.
Most of the bodykit will already fit into previous holes already drilled into the car where your runstrips are.
Heres a pic of my car whilst trial fitting my bodykit.
You can see the bit of tape below the rear arch. This is the bit that needs to be cut.