The McVities web site explains that:
"McVities have revealed that they created a biscuit munching robot to help with the development of their new Milk chocolate and Orange Digestive. With its mechanical mouth and teeth the Crumb Test Dummy has been specially designed to munch biscuits and produce crumbs in the same way as the average human."
There is a photograph. And the BBC gave further details, with another photo, in a March 8, 2005 news report.