Renegotiating rate for contract renewal

[m00]

Here is a situation, how would you negotiate in this position?

Worked a 3 month contract for a client, which was found through an agency. When negotiating the initial rate with the agency, they wouldn't budge and took a "take it or leave it" attitude to a figure which was 10-15%...

[Fandango]

Renewal time is coming up, I'm in quite a fortunate position presently in that i'm fairly critical to to the current projects success, so i'll be demanding a rate rise

I've found out what the agent is charging per day for me and realistically i can't see the client paying anymore...