heating is on but no hot water?

A LITTLE MORE DETAIL - SORRY !!
i have ravenheat RSF 84 E - HEATING OPERATES FINE ! WHEN HEATING IS ON AND I TURN HOT WATER TAPS ON I BRIEFLY GET HOT WATER THEN MAIN BURNER CUTS-OUT , WATER IS LUKE WARM FOR 2 MINS ( APPROX ),THEN MAIN BURNER FIRES UP AGAIN THEN SHUTS OFF AGAIN. WHEN HEATING IS TURNED OFF, I OPEN HOT WATER TAPS , I RECEIVE HOT WATER ( TEMPORARILY) THEN BURNER CUTS -OUT AGAIN !
P.S I HAVE NO INTENTION OF TRYING TO FIX THE FAULT PERSONALLY - I JUST WANT AN UNDERSTANDING OF THE PROBLEM SO AS WHEN AN ENGINEER ARRIVES WITH SLEDGEHAMMER AND STARTS SPEAKING SPANISH ,QUOTES ME £200 THEN PROCEEDS TO HIKE THE BILL UP TO £350 (LIKE THE LAST GUY) ... JUST DONT LIKE GETTING HABITUALLY SPANKED BY A SWEATY-PLUMBER WITH A SLIPPERY SMILE- NOTHING PERSONAL, IM JUST A HUMBLE JOINER WANTING A FAIR SERVICE :) THANKS