Cancel the half day and take the full day. Recently I showed interest in a 3 day a week cover at a school paying flat rate, and was about to finalise it with the cover manager when the agency offered me 5 days a week paid to scale somewhere else. No contest - I told the cover manager somewhere else was offering to pay me more and that was the end of it. Off the back of the second assignment I've secured a forthcoming long term cover. Follow the money - schools wouldn't think twice about dropping us to cut costs.