O/T and dull - IT contract agencies

I am sure some on here do IT contract work. Any recommendations for a decent agency please? Am looking to get back into IT after a break, so am looking for an agency I can work through exclusively either in the UK or overseas - I know there are some good ones out there who will listen to what I tell them, but some recommendations would be great. IBM mainframe analyst/programmer type stuff is my main area that I am looking into at the moment, if that makes a difference.

Thanks in advance!