I'd love to see other examples attached as comments to this posting.
Thanks to Keith Deane for letting me know about 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.
Keith does not relate HMRC's reaction to the claim - though I think we can guess....
Now that's novel. What other examples can you suggest?