The seat post diameter is 25.4mm so would that mean that I get a 25.4mm seat clamp (because it’s made for 25.4mm seat posts) or a bigger one (because the listed diameter is the diameter of the clamp and not the seat post it’s designed for).
All advice greatly appreciated 
Seatpost, seatpost clamp, and frame diameters are all listed as one size. So a 25.4mm frame takes a 25.4mm seatpost clamp and a 25.4mm seatpost and a 27.2mm frame takes a 27.2mm seatpost clamp and a 27.2mm seatpost.
Great thanks
What unihopper said is only the case if you’re buying a unicycle seatpost clamp. Bike seatpost clamps are listed by the inner diameter of the clamp.
If you are buying the seatpost clamp from a bike shop, bring your frame (or unicycle) into the shop, they will be able to measure your frame and let you know what will fit. If getting one from UDC you can just order one that is listed as for a 24.5" seatpost.
As has been said unicycle seatposts are sold by the seatpost dia. Bike ones are sold by frame size. This makes things both simple and complicated.
As a general rule, if your frame has a 25.4 seatpost and your frame is steel it will be a 28.6mm frame. If it is aluminium it is 30.6 or 30.8.
I hope that helps.
Roger