blown head gasket but it runs great just leaking a lot of oil

Hi, I have a 58 plate Fiesta 1.4 diesel. A few weeks ago i noticed it was losing oil. I use the car for long journeys every week and have had no problems at all. Anyway i lifted the bonnet and saw the leaking oil was coming from the pipes so obviously incredible pressure there. The coolant tank when opened was overflowing with black gunk. I called the garage and they sent a chap down to look ajd he immediately said it was the head gasket. I have had no other symptoms such as smoke nor water mixing with the oil so i am a little stumped! I suspect it may in fact be the oil cooler? The engine starts, no smoke and no overheating. Also no loss of power nor misfiring. Help please!!