I've had this case before the Commissioner on two separate occasions, the second occasion resulted in Commissioner Angus substituting the decision in the appellant's favour. I thought that I'd circulate it for others to read and, or, use. Paragraphs 15-19 deal with the de minimis principle, the nature of work and its interaction with Reg 16 of the Social Security (Incapacity for work) (General) Regs. 1995 and the evidential onus on the Secretary of State.

http://www.rightsnet.org.uk/pdfs/CIB_3914_2005.pdf