Because you're not specifically applying for employment which is exempted from the provisions of the Rehabilitation of Offenders Act, the agency can't ask you about spent convictions. (Or, if they did, you could still lawfully answer 'No'. Any other arrangement would make a mockery of the legislation).
However, as you recognise, some employers might require CRB checks and (at present) these always show all convictions. Unfortunately, if an employer becomes aware of a spent conviction there is nothing to prevent them from denying someone employment. (The agency would be allowed to completely strike you off their books, if they so chose, even though there might be jobs waiting where no CRB is required).
See here:
http://www.lawontheweb.co.uk/rehabact.htm
The CRB is currently developing a new 'basic' CRB check, to be used by most employers, which won't show spent convictions. (That's because campaigners have pointed out that the present system acts against the spirit of the Rehabilitation of Offenders Act). Until then, you're stuck with the imperfections of the current system.
The best advice is probably stick to only revealing what you have to by law. That means that you should keep quiet about your spent conviction for the time being. (If you get a few successful temporary placements with the agency, before a CRB check shows your criminal conviction, you're more likely to find that they're happy to keep you on their books).
Chris