Chevrolet Matiz when head lights r on break lights are on all the time, have just had mot?

Was a bulb replaced ? Perhaps wrong type fitted causing a crossover in the circuit is first thing that comes to mind ,would check rear light bulbs

Check if brake lights are on all the time when engine running , if so suspect faulty brake light switch .

Yes i would check brake light switch also check the switch mounting bracket i have known them come loose (2 bolts) this puts brake lights on as it moves switch away from pedal.