It's poor design I am afraid, and I contacted Dualit UK and US and they do not provide replacement viewing pane or rubber seal! I took mine out and cleaned them up, which helped some, but the limescale has eroded the plastic somewhat, so it's almost impossible to get a perfect seal again.
Turning the plastic viewing pane upside down (although defeating the purpose!) does place the compromised seal area above the water line and thus minimises or elimiates the leak.
Alex, December 2007
