I used to do 350 miles a week @ 45p a mile, and I found that it will cover petrol plus servicing - although not sure if you are saying your 100 a week is covered or just any extra you do off site. You will still find you'll have to pay for other consumables out of your pocket (tax, tyres, brakes, etc) but to me it was well worth the price for what you get. If you're careful you'll see just north of 30MPG. My old SEAT Leon wasnt half as nice and only gave me about 5MPG more (Leon FR 1.8T). And actually cost more to service!
Well worth it IMO.