The polemic article quoted makes mention of Wilberforce's mistress "Agnes Bronte". I think I have found the original website the information in this and other anti-Wilberforce essays comes from:

http://freespace.virgin.net/pr.og/agnes.html

This appears to be part of a spoof website with lots of spurious biographies of fictional Bronte family members but it has been taken as a serious source by people with an axe to grind.