A useful piece of legislation offers additional protection when you buy over the internet or phone.
It's called the Distance Selling Regulations and gives you a cooling-off period of seven days after you've taken delivery of a car.
This law covers internet purchases from a business, but not from a private individual.
Depending on the terms of the sales contract, you may also be covered if you place a deposit online or over the phone, then complete the deal at a dealership.
The law also applies if you choose the 'buy it now' option offered on some online auctions, but it does not cover you if you buy it as the highest final bidder.
If the car has been personalised or altered to your specification, then you're also not covered.