Renegotiating my rate

Hi - I need help!

My contract is coming up for renewal. I've been lucky to be on this contract for about 9months and they looking to extend it, for the third time. I haven't asked for a an increase at all. But I feel like due to the crazy nature of the work, and that I'm like the only original person left from when I started I kinda have the confidence to ask for an increase.

This is my first contract coming out of permie life, so this will be my first time asking for a rate increase. The guy said just to send an email.

A few questions - how does one go about asking? What metrics would you use, what type of support would you show, or do you just ask and hope for the best?

Should I benchmark other contract offerings (this makes me nervous because they might just say then go to those companies)...

And then what is the % or £ increase that is reasonable to ask for on a day rate? I know every contract is different, but just trying to gage here.

And is there anything else I should pay attention to or ask or be thinking about??

Thanks a million!!