Sam - there is a good example loan agreement on the BHS site - its pretty comprehensive and you can add points as you wish. I was considering loaning out my pony (until she got kicked and went down with laminitis so now she's staying here) and I took the agreement from there and edited it as chose.
Here's the link
http://www.bhs.org.uk/Welfare_leaflets/Loan-Agreement.htmI was very clear in the agreement about things like Shoeing (Maddie is barefoot and I didn't want her shoeing), usage, turnout etc etc
Make sure the prospective folk sign it and get it independently witnessed.
Also - don't know if you have a passport for the horse, but if you do you must retain it as proof of ownership.
Have read some horror stories, event between friends, so best do do it by the book.