As everyone else has posted it is an utterly bonkers idea and an extremely bad APR.
If your credit rating is poor you are probably better to look at financing the car through a dealership where the money is secured against the car. You won't get the sub 4% APR figures people are talking about but it will at least be single figures.
To put it in perspective you are talking about spending £340 a month on a 2 year old car when you can have a brand new Superb (just come out and a hell of a lot more kit and an overall better car than an E-class) for £240 a month:
http://www.lingscars.com/Skoda/Supe...ess_Hatchback_5dr_1968cc-personal.html#models
You can even contract hire a brand new E-class for similar money (note: I'm not recommending you do this!!)
http://www.lingscars.com/Mercedes/E...220_SE_Saloon_4dr_2143cc-personal.html#models
In your situation though I would be looking to borrow less money and go for something practical for you and the family, e.g. a 3 year old Mondeo, Superb, Passat.