I booked on line but was then was to phone them with credit card number which I did. I had booked for a month later, but the car came the next day. I never had a confirmation e-mail and was charged the full amount even though of course the driver never had to take me to Heathrow. I was not even at home the day they turned up. Would definitely NOT recommend.
I was kept waiting at arrivals for 50 minutes because the driver got the wrong terminal! This was in spite of the fact I gave them the flight number and correct terminal at the time of booking and it was even correctly written on my booking confirmation. The service cost £53 to go from Heathrow to London.... and they only agreed to refund me £6. I was pretty shocked - after all, getting the terminal right is a pretty fundamental part of a taxi service for picking someone up from the airport. Would not recommend I'm afraid.