There's many reasons I wouldn't buy from S20004U, but I'm struggling to see what's wrong with that car. Three and a half years old and six owners really isn't anything to worry about that much, as three of the owners could well be garages that bought/sold the car in the first place. Also, since it's an '06 it's not impossible that whoever purchased it didn't do their maths correctly regarding road tax and that made them want to sell quickly, or perhaps they just couldn't stomach the running costs fuel-wise. It's not hideously overpriced for the age and mileage (don't forget that the garage has to make a profit somewhere, so if they bought it for £12K then that's about right), and with the extra bits and pieces it's not a bad car for the right person.
Always buy on condition of the car first and foremost. S20004U does have a terrible rep when it comes to selling cars, I know, but if that was for sale anywhere else would anyone bat an eyelid? Unless the OP knows exactly what (if any) issues that particular car has, then I'm inclined to say that there's precious little wrong with it as it stands.