I've recently come across this contract for care workers which to me seems unlawful. I might be wrong about that, but at the very least it's pretty unscrupulous.
As it's a zero hour contract the employer is not obliged to offer a minimum (or maximum) number of hours. However, employees ARE obliged to work whenever they are told to. ACAS states that zero hour workers are not obliged to accept work, but this company states it's a condition of employment. Elsewhere the government suggests that it should be made clear to zero hour workers that they do not have to accept work.
In another part of the document it is stated that pay is determined by the number of minutes workers are "logged in to client's homes" and not for time travelling in between. Again, ACAS seems to suggest this is unlawful.
The cherry on the cake for me is their recruitment poster which boasts "flexible hours", which I suppose is true from the employer's POV, just not for workers.