A dealer (or indeed a private seller) has to provide a roadworthy car. That isn't the same as a faultless car.

Selling an unroadworthy car (e.g. tyres below legal limit, brakes not working etc.) is a criminal offence. A new MOT is usually a quite good (but not 100%) defence for this.

Without some of the issues detailed it is difficult to comment more.

Very difficult to prove after the car has been used for a month,

When bought from a trader, the onus is on the seller to show that the fault didn't exist at the time of sale, unfortunately for the buyer, an MOT carried out at just before the sale would probably show just that. If teh MOT is older, ie months old, you might have a case.

Edited by Hamsafar on 31/10/2013 at 17:45

There is a very fundamental difference between unroadworthy and a fault.

An unroadworthy car is a criminal offence to sell and the seller, be it trade OR private, is liable to prosecution. Unroadworthy means bald tyres, broken suspension, bad brakes etc. It does NOT mean a dodgy headgasket.

An argument over a fault or other aspect of the cars performance is a civil matter between seller and buyer and open to endless interpreations and comes under the "how long is a bit of string".

An actual unroadworthy car at point of sale (e.g. illegal tyre) is far more cut and dried.

Yes, where is the OP, by the way?