It was the old washer. When I pulled the valve the washer stayed in. It wasn't going to come out - I just pushed it and it moved off to the left of the hole. The new valve is working fine.
Thanks.
I am changing the cold water tap's valve body on a Paini Kitchen mixer tap (about 12 years old).
I've unscrewed the valve body from the main tap body and the valve is loose within the body, but I can't seem to pull it total free. It appears connected to something from below. What could be...