How can i fix lock on boot of renault megane convertible ?

The boot of my renalt megane 54 convertible has suddenly stopped locking which means the whole car will not lock, and every time i accelerate from standing position the car beeps to tell me the boot is open. Again renault want to charge £83 pounds to diagnose problem, i know the problem, it doesn't lock.
Would never buy Renault again, car not been too bad but customer services are appalling.
Please help