In addition to the MINIMUM 9 pounds fee (which has to be booked on the internet by the way), there is as Dudster has pointed out a 5 pounds cleaning fee.
Other points to consider are that there is a daily mileage allowance of only 75 miles (100 km) - mind you in a Mercedes A class I wouldn't want to do much more! Also there is an insuarnce excess of a whopping 500 pounds!
If you are considering the London Bridge location, be aware that it is on the top-floor of a multi-story car park, within SIGHT of London Bridge.
I think Dudster is mistaken when he says it is not targetd at the business traveller, but I agree that it is not suitable for the business traveller - like everything in life I believe you get what you pay for, and in this case, like the airline, I think you will get poor reliability.
If I was on holiday I think Easyjet would be great, but for business the limited schedules, lack of replacement aircraft and inability to transfer tickets are a massive hadicap. I have barred people who work for me from using the airline, the cost davings were sometimes great (although not always!) but the number of missed meetings was unacceptable.
I think the car rental business will be the same.
Nick