This sounds fairly contractual to me, rather than strictly h&s. They're your employees, so you have a duty of care. They're a hire company, so they have a duty of care. You're asking good questions but the devil will always be in the detail of the systems/cabin vs location and previous locations - and there aren't any specific legal requirements around this in relation to legionella. In practical terms I'd like to know that the cabins were being *fully* drained whilst not in use, then preferably superchlorinated prior to supply. I'd extend that to the showers and CWST's where present too - because I wouldn't trust previous users to necessarily be careful with regard legionella. So that's what I'd look for evidence of as part of my decision to hire. You can look for micro sample results if you like but they're easy enough to get looking good if you time the sample immediately after a chlorination...so might be of limited use.