Diplomat freezer beeping and motor keeps cutting out?

Got a Diplomat integrated freezer, defrosted it yesterday, when I put it back on it beeps and the red warning light flashes, motor runs for about 20 seconds then stops, pressing the fast freeze button stops the beeping, but the motor still cuts out after 20 seconds again. It will be a year old end of this month, but Domestic and General will not honour the warranty and send an engineer.
Someone suggested tapping the black compressor at the back, as sometimes the piston can stick, tried it loads of times, still makes no difference.
Has anyone else had the same problem and do they have a solution?