98 E200 locking problem

[screwballpaul]

Hi there just bought a e200 but only came with the keycard ike a credit card and small key . It starts the car fine but when i try to lock the car the key turns left and right but wont lock the car. i had a look to see if anything had came off the locking system but couldn see anything. Was told by merc i needed key fob and key £300 to lock the door. But the key in my eyes should still lock the car.

Any help i would be gratefull.

Thanks