Wiggy, you are correct to a point, duty of care does primarily relate to health and safety legislation including safe places of work etc, etc.
It does also cover employees welfare, so not just ensuring that you kettle has been PAT tested!
I do not pretend to be an expert on contract law, but I am an employer so do have first hand experience of this sort of thing.
Where your conditions of employement permanent or otherwise can be proven to be detrimental to your welfare, then there is certainly a case to answer here, albeit not clear cut I admit.
Stuckgear you are not entirely correct there I am afraid, contractors when at your place of business are considered as employees for insurance purposes, they are also considered as such by Hmrc if the contractor has no other place of business.