How about deciding on what you would expect for rent/livery then work out how many hours work would be needed to pay for the rent. Once that is sorted you could say any extra hours worked would be paid at an hourly rate. Also just a thought is the room large enough to rent to a couple. This would widen the group of people you can choose from.
I didn't have a written contract. Yard owner and I discussed things like wages etc before I moved in, basically she worked out how many hours work I would be doing and so how much i should be paid, then took £30 out of my wages each week in rent and she also charged me for livery (though at a reduced rate), so what was left over was what I got paid per week (not terribly much!). There were lots of perks to the job though, my horses were opposite my window so I could sit and gaze at them if I wanted! YO took me to competitions with her nearly every weekend. She offered to cook me dinner each night, though as I am an extremely fussy eater, and vegetarian which none of her family are I felt it would be too much hassle for her so declined. I really enjoyed living in, yes the caravan was very cold in winter (and most of the year!) but it was nice to have my own place. (Had just left uni so my first time properly working full time away from home). In the end I got offered a supposedly better job (it turned out to be awfull but there you go!) so moved on. But I have many fond memeories of working there. As someone else said I think your job would really suit someone at university or prehaps someone who is looking for somewhere to rent and keep their horse and just wants to work part time. It sounds good and I hope you find someone!
Thanks so much for all the help and ideas, I think I know pretty much what I'm looking for/have to offer now. Will look at placing at ad on yard and groom or somewhere similar and see how it goes. Thanks again for all the advice!
xxx