Paul,
Reasonable questions - could be a huge response though...
"Steve, also do you know if you HAVE to issue a contract to a contractor or can you just issue them a summary of conditions? I read somewhere that a summary of benefits and conditions would be OK."
Can't comment on employment law but as a contractor I would only work to a signed contract.
From the contractor employer side implied conditions apply once the contractor walks in the door on the first day but that is something only resorted to when needed.
Re the fees etc, I see this a new field for you ?
Do you have IT experience but not of contracting ?
Do you have any experience of dealing with Agencies ? ?
If not then how do you propose to find the vacancies and to get your contractors in the frame ?
In any regard there is a huge variety out there. many agencies will exploit both the contractor and the employer. And there is a lot of tricks they use.
Many (including some of the household names in the industry) will, in my view, put forward the contractors that give them the biggest payback and not the best person for the job. The agencies have a host of tricks to achieve this. I am sure the many IT folks on here could comment also.
Experienced contractors know the right questions to ask to see through dodgy practices and can do so very quickly - at least I like to think I can anyway

If you took 25 percent of the fee (or more) I would not work for you !!
Best by far is transparency and a fee at a fixed percentage - 6 percentish is reasonable I think. That way you are quids in with the contractors and can build some loyalty and quids in with the employers as they they know your contractors will know the full financial facts up front and won't up sticks and leave when they find out they are getting stiffed by some greedy agency. Nice warm feelings about staff not walking out the door willy nilly are worth something - quite a lot when its key project stuff.