hi, can anyone help? i have a baxi 105e, the problem i have is this,

anytime i try and switch it boiler on it try's to fire up to only then show a flashing red (fault on pump) light.sometimes it comes on after three or four attempts sometimes not at all.the hot water doesn't come on at all.

had the circut board changed, new heat exchanger ,new pump and still wont play . one guy said i have a blockage in the system as the water leaving the boiler is at the correct temp, on returning is much colder,
i have drained the system a load of times, flushed it with fernox f5 and its pretty clear to me, i would appreciate any input as i think i might be missing something, thanks. sam