There is no obvious source for this story on the channel 4 website. But it does provide the name of the Indian lover - Giridhari Mehter. That name appears on only two websites (that the major search engines can find), the channel 4 website and this review of the programme:
http://www.thefreelib...+solution.-a075286226
I have to agree with the reviewer: the treatment of the actual evidence (the letter which apparently revealed the truth of the case years later) was derisory. And you would think such a story would be better known and documented. Can anyone help?