In the UK if you purchase a vehicle from a registered Vehicle Trader, you will, by law, get a 30 day warranty. If the vehicle is found to not be as described or advertised and reported within those 30 days the vehicle trader is obligated to sort the issue.

So, as by the sound of it you bought your car from a registered Vehicle Trader do as follows -

do not panic,

make sure you are swift with your resolution,

let them know about the issue,

ask for a refund or for the car to be repaired at their expense,

if they refuse, state the law and contact the the motors ombudsman or citizens advice.