Hi Martin, this is the type of job where you'll be competing with the big boys and maybe also, at the other end of the scale, the cash in hand brigade.
For what it's worth, unless this is a space filler, where you're looking to add hours to staff already in the area, or maybe a route to much more work then I'd personally steer clear.
You can bet they'll be looking at a max of £12 per shift and odds are it'll be 5 p.m. or there abouts. Really not worth the grief as a stand alone.
Main issues will be getting someone to agree to work just one hour, management time, covering sickness.
It may be worth trying to talk them into a different arrangement.