Zanussi Washing Machine E20 fault?

Hi
My Zanussi washing machine keeps getting the E20 fault. I have taken out the front filter thing and cleaned it and it still happens. When it errors I push the reset button and it works again but only for a matter of time before the E20 shows again. Is there anything else I can do?? Is there another filter that could be blocked?? If so - how do I empty/clean it??
Thanks
Vix