1991 Vauxhall Astra Oil in coolant

Hi, wondered if you could help me. I brought a car of a friend from work on the cheap as he got caught drink driving. I had it serviced the day after and MOT'd, passed with flying colours, the mechanics told me the water tank was empty and they filled it, but it had signs of oil leakage in the water tank and to keep an eye out. Its a 1.7tdi, and it had a new head gasket in 2006 on 121,000 miles. I just checked again today and there is black oil in the water tank now, and I really dont know what it is as the car is not overheating at all, as I know with a head gasket the temperature goes out off the scales and you have to keep replacing the oil and water. So I am kind of stuck as what to think it is and is it better to get rid before the car completely breaks down. I have asked the mechanics to flush the water tank tomorrow and refill. What other options? Or are their any other ideas as to what is could be?