felix said:
OK, perhaps it depends on how you look at things. the client could pay the agency £400 a day for you, and the agency could pay you £300 a day. that extra £100 a day is pure profit for the agency, and they are free to change that if they so desire. that is exactly what happened to a mate of mine, and he didn't find out till about 8 years later. the agency lied to him about how much money they were making out of him.
i'm in the UK, if that makes any difference.
I have no idea how things work in the UK, but at my agency (we are a premium agency) we charge 19% for any casual placements. We do negotiate lower rates if our clients offer us exclusivity for a role, or if we're on their preferred suppliership list.
I can think of one specific example where a client of ours wanted to hire a candidate, who was asking $50k a year. This particular client wanted to lower their costs in general, and since we weren't prepared to negotiate our casual fee card with them (it's a casual card for a reason), they requested we offer the candidate 45k instead.
We informed the client we felt that this was unethical - we were representing the candidate at their asking salary of 50k, and if they wanted to negotiate otherwise, that would be between them and the candidate. The client kicked up a bit of a fuss and told us they would just use another agency, and we'd miss out on a placement altogether, when all the work was done.
We didn't. Three days later, the client got back to us... they wanted our candidate, they paid us the full 19%, and on top of that, they're now wanting us to be a preferred supplier to them.
It just goes to show how much a client will respect you, if you're prepared to lose all your hard work, for the sake of good ethics!

We will
never take a cut of our candidates salary, even if the client suggests it.
Stickygreen - will respond to you when I have more time.