Does anyone know this book?

I read a book about 5 yrs ago, and cannot remember the title. Does it sound familiar to anyone? It starts in the early 1900's with a young man, possibly a student at either Oxford or Cambridge, boating on a river and has a collision with another boat. He goes to tea with the people from that boat and they are a Russian family. I can only remember vaguely what happens from here - he falls in love with one of the Russian women, who is married to an important Russian. Somehow they get together and end up in Tibet, where the student is a monk. It turns out that the English student is actually a spy. It all sounds very odd, but it was a really good story and I would love to read it again, but have absolutely no idea of either title or author. Any suggestions gratefully received. Thanks