How can I fix my Brandt upright freezer?

How do I tell if the compressor on my Brandt upright frost-free freezer is faulty? The freezer has stopped working now but from time to time it sounded like a motor tripped in for a few seconds then stopped. A company I chose from yellow pages came, told me it was the compressor and wanted two arms and a leg to put it right! If it is the compressor, how big a job is it to carry out ?