Ria, this is not wholy a dealership problem, it is a manufacturer's guarantee problem. Check the purchase Ts & Cs the conditions in the guarantee and also the car insurance, be it separate or in the purchase price..
If the insurance was part of the purchase price get in touch with them and explain. Also get in touch with the manufacturer as I feel sure that a technical breakdown in three month would be a cert for an immediate temporary replacement.
Also contact Citizen's Advice.
What are the symptoms of the fault?