How can I fix my Saunier Duval Semia Condens boiler?

The Central Heating (CH) side works perfectly but the Domestic Hot Water (DHW) has developed a problem, thus... Turn on hot tap, boiler lights and after a few moments hot water starts coming through tap. Temperature indicator then starts to drop and water temperature falls from very hot all the way down to cold if hot tap left running. Boiler still working (I can hear the burner) but temperature indicator varying between about 39 and 43 although temp knob turned to max. No fault code indicated but burner keeps turning on and off until hot tap turned off. Can get only a few litres of hot water, probably about 5 or 6 Ltrs before temp drops away. Any ideas? Can I fix this or must I call in a technician?