OP, I think what NLUK means is that due diligence extends beyond checking the pieces of paper that link you to the client and getting them reviewed.
Consider the following:
- collect any and all evidence of you being treated differently to a permie because you're a contractor. For example, social nights out where contractors have to pay but permies go free.
- think as a supplier, not an employee. Are you being paid to do that piece of work they've asked of you because it's in your contracted deliverables?
- behave as a third party consultant; promote brand you. If something isn't in your deliverables but they talk to you about it, explain politely that it's not what you were brought in for but you're happy to quote for it as part of further deliverables and the priorities of that versus your original contracted work can be negotiated too.
- don't take up any employee freebies, no matter how tempting. I don't mean the free pens that end up on every desk, but more the employee discounts that can pop up in canteens, etc.