Hiya,
Welcome to the site.
May I suggest
www.officerecruit.com for finding an agency in your area?
The best nationwide agencies that I've found are Office Angels, Pertemps and Adecco.
I prefer local agencies only because they tend to be smaller and much more pleasing to their clients. But that's just my opinion.
From my experience with temp agencies, I've found that you have to keep reminding them that you're there. Keep calling them. This keeps your name in their minds when they're looking for a worker. You want to stay on top of their pile to get the best jobs. So I called at least once a week until I bugged them into an interview. If you're getting the run around from an agency to even come and sign up then you're better off going to another company. If they can't take the time to meet you, they probably won't be good at getting you a job.
Join Monster.com. I've had a couple of agencies contact me because of my cv on the site.
Go to every agency you find. I didn't do this in the beginning, so I relied on just one company and they really screwed me over. So I went to every recruitment company in the LS1 area (Leeds city centre). The response was immediate. I would get calls about jobs 2 to 3 times a day.
Good luck with it all. I'm sure you'll do fine. They love our accents.