Its the Millenium Episode... I havent seen it in ages... buuuut...
Diana, I presume, comes to spend New Years Eve with them, and then, she smells a "gas leak" and it gets *beep* up... they have a horrible time. Then Christine comes, finds the "gas leak" which is actually, some old fish fingers. Bill takes Diana oustide, and the people roit around her. Then it ends. That was the last ever episode. Gary Olsen had already got cancer half way through the last series, but wanted to finish the last one... I think.
It's a shame the series ended like that, cut off like that but I can't see how it could have continued after Gary Olsen's death. It wouldn't have been the same without him. As a sidenote, I remember seeing that last episode and thinking it was pretty unfunny. By the eighth seires It was beginning to go off the boil and they really should have thought about wrapping it up.