We have reached The End.
This adaptation was based on Curtain: Poirot's Last Case, first published in 1975, just a few months before Christie's death, but written during the war, in the early 1940s. The novel was adapted for television by Kevin Elyot (who also scripted Five Little Pigs and Death on the Nile) and directed by Hettie Macdonald (who also directed The Mystery of the Blue Train).
Script versus novel
The press pack to this final episode reveals that Kevin Elyot was asked by the production team to adapt Curtain more than ten years ago, when he wrote the scripts for Five Little Pigs and Death on the Nile. I'm not at all surprised they asked him. His script for Five Little Pigs is possibly the best of the entire series, and Death on the Nile proved that he wasn't daunted by the task of writing an adaptation most Poirot fans have been both eagerly waiting for and dreading at the same time. He had also demonstrated that he fully understood Poirot's character, and that he could handle the darker side of Christie without making unnecessary changes. In my opinion, he was the best man for the job.
SPOILERS. IF YOU HAVE NOT SEEN THE ADAPTATION YET I WOULD ADVISE YOU TO WAIT - READ THIS POST AFTERWARDS.