Zanussi in Brentwood ?

E20 during rinse cycle?
During the rinse cycle when water should be draining from the washing machine you hear continuous beeping and E20 flashes on the display. The only way you can continue is to drain the washing machine manually. The filter has been checked (accessible from the front) and all the bits cleaned out of the filter. All of the internal drainage pipes have been checked and cleared of any bits (found a few pieces of straw from our washing basket). Ran a test cycle and the washing machine still comes up with E20 during rinse cycle. Removed the external drainage pipe from the U-bend waste pipe, still get error E20, however, if you keep the external waste pipe at floor level water drains out and the rinse cycle works without errors. It is as if the waste pump doesn't have enough power to force the water out of the washing machine. Please can anybody help? Thank you.