Agencies I would recommend working through.

[rob s]

Resource Solutions +1
JM Group +1
iSource +1 for prompt payment and being good to work with, -1 for doing their homework on the role being signed off on one contract (though I assumed it was so feel free to discount the -1 for older and wiser contractors who do ask this question...)
Pink Elephant -1 (30 working day payment terms) and late on more than one occasion

[ThomserveBAS]

5 (almost 6) months into first contract taken via Rullion and they have been great (in that I've never had to actually deal with them since starting my contract). Paid weekly, no hassle, no fuss, and (having slept since then) the contract signing was very easy too.

[SeekingIT]

JM Group

craig1, follow up from your post about business relationships...finished a contract with JM Group back in Dec. I'd definitely recommend them in terms of payment (always paid out around the 20th of each month, and the accounting dept emails reminders for invoices if they haven't received them from you) and advice (the transition from umbrella to ltd was seamless with my recruiter). I still get regular calls from her regarding my availability and potential contracts which is definitely nice to have.

COAL IT on the other hand, alright contracts but chasing up for payment once the last contract had finished was a major issue.

+2 for JM Group