Can anyone explain the ending?

Me too. I think it was supposed to mean that it was all in his head. It's as if he suffered a stroke or something that made him a vegetable and in order for him to entertain himself he had to make up the whole story and live in a fantasy world, and the scene at the end where his friend in the safari outfit comes to get him is to represent that finally he has died and was given peace. I think that's why the frog also winks at the camera; as if to say: "this was all a fantasy, got ya!" That's my take on it. Especially when you consider that the whole film is narrated, it's as if it's the voice in his head making up the whole story. He probably did have a family, and was a gentleman of the aristocracy, but the whole rest of the story was just him being stuck in his own mind and needing to entertain himself while he awaited death. A similar film to check out would be the French film (and a true story) "The Diving Bell and the Butterfly." It has a similar story of a man who suffered a stroke and could still think but couldn't express himself in any way except through winking, and what he thought about while he was in a sort of conscious vegetative state. Hope this all helps.

I think the poisoned pipe's disappearance meant that Fledge (Sting) did pick it up after finishing the dance and most probably met his end soon after his incapacitated master's passing.
The butler got his comeuppence, and even though Sir Hugo wasn't there to see it and enjoy it, it were his personal notes that facilitated Cleo's poison scheme. In that sense, he did get his revenge, though from beyond the grave. The toad's wink might indicate that, as if to say: "we sure have fixed him [master]".
Needless to say, this scene was a bit ambiguous - it was the toad's wink that threw me off initially. Maybe they should have established the toad as its master's favourite pet or something? Or did I miss that? Anyway, it probably could have been done better, but overall I was quite pleasantly surprised with this little film.