200 guests? Okay, that's a wedding-sized event.
Get her expectations ASAP. Is this a birthday party? Are there going to be important moments during the night that she wants captured? Does she want formal photos? Or is she just looking for someone to grab candids all night? Does she require photos of every guest? Does she expect you to be up and on your feet for all 4 of those hours or are you also attending "as a friend" AKA taking part in the festivities?
When there is money exchanging hands, definitely find out what she wants and get it in writing. It's not that you don't trust each other it's just good to have a formal understanding of what each person expects.
If there are any expectations above "get candids of whatever" throughout the night, I'd suggest no less than £80/hour to pay for your time being there, equipment rental, plus the time spent doing any editing after the fact. And I'd rent the 16-35 f/2.8 over the 17-40 because the 1-stop faster aperture will help your recycle time for your flash and/or let you shoot at a lower ISO. But that's all assuming she wants some kind of structured photography. If it's a candid-only, low-expectation kind of job then you could likely get away with what you're using now and charge her £120 for the night so long as they feed you
I'd still get it all in writing, just so if mum&dad come wondering where their money went they'd know.
Just my two pence
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