I'd love to see other examples attached as comments to this posting.
Thanks to Keith Deane I've just found out about the a self-employed accountant who worked from home. He claimed that he often received books from clients (especially jobbing builders) that were "covered in muck".
As a result he claimed that he had to constantly wash his hands to rid himself of exclusively business dirt and as such the water so consumed was an essential element of his business activity. Thus the accountant claimed a proportion of his household water bills as being business related.
Now that's novel. What other examples can you suggest?