I read this novel several years ago (Nov 2010) and was very disappointed. But I was in error. I just reread it. Helena is delightful.
The trick to enjoying Helena is to prepare for a very different kind of Waugh novel. In fact, a unique one. Most of Waugh's book fall into natural groups. Decline and Fall and Black Mischief and Put Out More Flags -- these all hang together. The travel books (at least the three that I've read) hang together. So do Brideshead Revisited and Sword of Honor, and again The Loved One and Love Among the Ruins. But Helena stands alone in his oeuvre.
The first two-thirds, ending with the grisly murder of Fausta, are uniformly excellent. In the final third, the story scatters. We depart from Helena's perspective and ambit. Perhaps this is appropriate in that we have finally reached her mythic achievement, but after the beautifully rendered first eight chapters, it's a shock. I'm don't think it an excellent conclusion -- though these chapters contain a wonderful discourse on the Magi, and several moving religious passages.
Simon Prebble's narration (Audible) is very good, but I recommend reading along with him to catch certain nuances that he elides.