Have you looked at the HSE prosecutions database, especially breaches under H&SAWA 1974 section 3-1?

R v Associated Octel is the oft-quoted case law. Closer to home (for me) was the contractor fatality at a local paper mill. See (e.g.) http://www.bbc.co.uk/new...burgh-east-fife-12543314 In that report you'll see that "court heard an employee of Tullis Russell had failed to follow proper procedures when hiring them". Individual prosection aside, you could perhaps ask your own people whether they think that employee stil works there, and how the life of that person and others have been affected. There is of course a wealth of info. on HSE website on managing contractors, and the EEF (amongst others) run specific courses, however it looks like you need something hard hitting to focus the minds of a few individuals.