Best place to have car error checked?

[twoblacklines]

So now my £5.5k, 1 week old (in my ownership) 330ci is now in limp mode misfiring all over the place (except no engine check light is on or any other error light for that matter) I need to find out what is wrong with it.

Any ideas the best place to take it to? Should I just go to BMW? Will they help as it is out of warranty? How much will they charge for a diagnostic?

Thanks

[twoblacklines]

The place I bought it from claims to be a specialist: http://www.specialistvehicles.co.uk/service yet they seemingly missed all the faults this car has on their "120 point check" they did before they sold it.... which doesn't give me much faith in "specialists".

[twoblacklines]

Thanks, I will bear that in mind if it gets worst, at the moment it only misfires on acceleration under load and not idle.

[twoblacklines]

I have had it about 10 days I think. Isn't SoG act only 7? I haven't had time to take it back during the day as been too busy.