Tell them you are happy to help but need to get a written contract for six months drawn up. Explain they will have to pay the fees for this to be done (watch their faces, it will be an indication of how keen they are). You will need to have it looked over (they should pay for that too), and then tell them once that is all in order you are willing to continue the arrangement. Make sure the agreement says who is responsible for what, inc any damage or 'putting right' after they vacate. This includes reseeding poached areas and importantly - insurance.
[/size]If this isn't agreeable to them, close up the access hole again and tell them politely you understand and it's bye bye grazing.
[/size]It's not worth the worry. [/size]