I've been completely obsessed with both the book and the TV series since 1992 when I saw the latter, which led me to the former.
Yes it was Ehle's big break, as it was for Maggie Smith's son Toby Stephens, Brian Walden's son Ben Walden, and Tara Fitzgerald. Ehle's mother Rosemary Harris played the older Calypso, as part of an ensemble cast with everyone from Paul Eddington, Felicity Kendall, Claire Bloom, Richard Johnson, Nicholas Le Provost (as both Hector and his son Hamish) Virginia McKenna, and a 10 year old Rebecca Hall in her father's dramatisation as Sophy.
If you're interested, the house used in the filming is very much still there, on the Veryan peninsula in Cornwall. I'm lucky enough to have a little place nearby and am there a lot. There are new people living there now but the previous inhabitants were the couple who loaned the house to Channel 4 for filming. They took it on in the late eighties from the National Trust and it was a money pit, they were completely skint when Channel 4 turned up one day and asked if they could pay them to film, which was a godsend. They had small extra parts in it, you can see them as the kitchen staff.
A decade or so ago I spent an amazing afternoon with them. I was skulking round the side of the house on the clifftops and they invited me in, gave me a home made cream tea and got all their scrapbooks out. I saw photo after photo taken during filming, the cast at work and relaxing. It was one of the best days of my life 🙂