I have signed a contract but, probably like most people's, it's a little vague.

I like it that way.

I always make sure there's a line that basically reads "and anything other job as necessary" in my contracts. It gives me a lot of scope for doing other things when I have the time.

I think contracts are a good idea for working hours, working conditions and things like that, but specifying actual duties may be a step too far.