Can anyone identify this book please?

I posted this query a few weeks ago on a BookCrossing forum, and, thanks to other members, some suggestions were made but the book still eludes us.

My Mum would really like to re-read a book she read some 30-40 years ago. For her upcoming 87th birthday, I'd like to be able to track it down for her. She can't remember the title or the author but I hope someone might be able to help.

Synopsis: Set in the UK around the 1950s/60s? A bunch of nuns have had to leave their convent and relocate to a very rural location, in the middle of nowhere. However, these nuns were very good at living off the land and were expert herbalists. The herbal medicine aspect seems a significant theme in the story. They take in an escaped convict/murderer who's in a bad way, and they nurse him back to health, whereupon he tries to take over and bullies the nuns, and threatens the Mother Superior with a gun (we don't know where he got the gun from though). He doesn't shoot the lady boss but eventually gets his come-uppance in the end.

Please post any leads