What is where the patio has to be laid? Earth? Needs to be hardcore/sand and dabs of mortar, really. Have hardcore?
You really should lay a concrete base for the shed, unless you buy one with a floor. In that case, it is laid on supported timbers. Earth boards around.
This is why they are coming around to quote .. to see what's actually needed.
I think the guide price above included the shed base as well. Without knowing what size of shed base(say 6'x8'), or what preparation will be needed in both cases, I wouldnt think that £750 would be wildly out.
Good luck hmp and good luck to your builder/landscaper. Assuming some prep is required on the 10sq.m of work, then the material cost inc a skip and plant should be around £350.00.