ignition problem

i had to purchase a new key fob 4 months ago i was shopping on sunday when i returned to the car i could not gain entry to the car i called the rac out and he disconnected the battery in the boot of the car and disconnected it for 10 mins but to no avail the car would not start the car is now in mercedes dealership in my own town but ive got a funny feeling they are going to sting me as anybody else had the same problem cheers john oxley