How can I fix my Zanussi ZWF1632W Aquacycle washing machine?

Hullo there. My problem is - I took the filter out to clean it and after putting it back, found water just flooded out of it when I next went to do a wash. I've taken it out and replaced it several times since, with the same result. The rubber washer seems to be 'free-floating' - not attached at the top or bottom or even in the middle - when I try to fit it firmly round the top, it doesn't stay in place and anyway, I can't get the filter back in! Can anyone advise, please?