The Welches withdrew and began getting into their car. Moaning, Dixon allowed Christine to lead him away up the street. The whinnying and clanging of Welch's self-starter began behind them, growing fainter and fainter as they walked on until it was altogether overlaid by the other noises of the town and by their own voices. (25. 66)
How sweet is that? Jim Dixon is leaving the train station with the beautiful Christine Callaghan on his arm and a new job in London to look forward to. But just when life can't get any better, he runs into the entire Welch family on the street, including Michel, the son we haven't met until now. Amis really goes out of his way to gather all of the Welches together for Jim's great moment of victory.
Jim's moaning because he's so overcome with joy and spite. He laughs in all of the Welches' faces. He wins, they lose, and it's as simple as that. It's also a symbolic victory, since the ending suggests that Jim's loose but honest style of living has defeated their pretentious posturing. The final lines really drive this idea home, as the sound of the Welches' car dies away, leaving only the sounds of Jim and Christine's voices. They're the last ones standing.