how can i fix my anti pollution fault on citreon c5 hpi?

had anti pollution fault and loss of power, took to main dealers they done a diagnostic and told me it was the down stream sensor [lambda] . they changed it at an expensive cost. when i picked car up i did not get 200 yards up the road and bang same problem, took it straight back.after 2 more days they told me it is now the high pressure pump and will cost £600. i told them not to do it,can the fuel system be flused through or will i need this new part.