Definition Issue?
I suspect that there is a definition issue here.
Mr Angry is probably in the right area, and ZAV is correct also. ...............although those agencies that claim not to employ you - calling you "self employed" do sail close to the wind.
Mr Angry's defined min of 28 days paid is spot on.
Different employers define days differently................a "normal" day off for one employer might be defined as a day's leave by another.
Employees faced with this situation need to keep detailed records - not just against the 900 hours, but about their days "on" and "off" and how they appear to be defined by both the AOC holder and the agency............and if what you are told doesn't sound right, put it in writing to the employer.
In the end, you might have to pay for some advice.