I've read three of the Mandeville books, in order, so this is more a commentary on all three.
First, I love the premise, because I love a good time-travel romance! And the first one ("A Time for Change") in particular fits the bill. Of these three books, I think this one is best. The author sets up the framework for a good series, and she doesn't deviate from it. The means by which her characters travel through time is complex, but she is solid in how it works throughout all three novels. It would be easy to cheat a little in how the travel is handled, but she keeps it consistent.
Next, the characters are interesting and enjoyable, for the most part. The main female character (MFC) in each is a strong person who has been beset by a deep tragedy, which is important in how she comes to the Mandeville estate. (I have a few quibbles with Nell/Helen in the second book, "The Mandeville Secret," because she became a wee bit annoying, but I might be a little harsh on her because I, too, have had writer's block!)
My biggest complaint about the series is that books 2 and 3 are too long! I felt like I was living right along with the characters because every single activity felt documented. Every cup of tea that was poured, stirred, drunk and enjoyed. Every fire that was laid in the hearth. Every meal that was prepared and served. I like detail, but this was just too, too much. These books needed a good editor to trim out some of this excess.
While I enjoyed the third book, "The Mandeville Shadow," I felt like it took too long to get to the heart of the mystery. Not until a good third of the way through the book did we get a handle on what made this book a mystery, and find out what the shadow and secret were.
I will probably go on to read book No. 4 in this series, but needed a break from time travel so escaped into a romcom ... only to discover that it, too, is a time travel story ("Cassandra in Reverse"!)
If you enjoy a very British novel with a little mystery, a little fantasy, some healthy doses of romance and a credible time-travel plot, give the Mandeville series a try. Just be patient, because these characters drink a lot of warm beverages by the hearth!