worcester heatslave 15/19 combination oil boiler?

hi we have had no heating now for 3wks could anyone tell me whats wrong with my boiler i have hot water but no heating .when heating is switched on it gets hot at the radiator valve end on the rad closest to the boiler then it goes no further.we have had many new parts on this boiler it was practically rebuilt last year by worcester.we have had a plumber out he put a new pump on but still no joy he says the motor is working fine the only thing that he can think is the radiators all need flushing out.can any one help or had simalar problem as this plumber charges £40.00 an hour!!!!!!!!!