I've just bought a new Silvercrest Microwave from Lidl for £79.99. I've got home and plugged it in, but the microwave functionality doesn't seem to be working. I put something on for three minutes on 900w and it came out stone cold. When we first turned it on, we used the grill setting for 10 minutes to clean it, as instructed in the manual and this worked. The elements lit up and it became very hot. Am I doing something silly or is it possibly broken?