Una danza para la música del tiempo

It's possibly a bit late to post this question but as I have only now managed to see the video, here goes anyway. Does anyone know WHY it was deemed necessary to replace James Purefoy and Emma Fielding as Nicholas Jenkins and his wife in the last film of the series? Most of the other characters were left to age, convincingly or otherwise, even Widmerpool himself. Though Joanna David did at least bear a tolerable resemblance to how Isobel (Fielding) might have looked in later life, John Standing, excellent actor though he is, didn't look remotely like an aged James Purefoy. The changeover broke the continuum of events for me and was a constant source of irritation. What was behind this strange, irrational decision?