How can I fix my mira advance standard - fault - low pressure then cut-out?

can any body help?? after 30 seconds or so my mira advance keeps tripping the internal relays whether set on low/med/high, then displays low pressure before cutting out. I have a sealed system combi boiler with 2 bar pressure, the shower head is clear (although i do live in a very hardwater area). I have checked the inlet filter (clear) the turbine (clear/freely rotates) there does appear to be a tube type filter in the flowvalve assembly which looks a bit damage.
Update, ive just put it all back together the relay fired immediately and now there is no water flow at all - i bled it for a minute!! doh.
Any help would be gratefully received.
Thanks
Tony