I work for a couple of estate agents.
Nothing in writing, they just give me a call when a job comes up. Its always properties that are let out. I clean the properties up, usually for when the new tenant moves in.
Window cleaning, gutters, conservatory rooves.
I charge 30 p/h (south-west prices). I charge for all travelling time at this rate. As the work is all in my town, I don't add anything on for fuel.
If I were you I'd get a professional looking pack together and just pop in one day. I imagine most will already have someone though. I started as a fallback option to another window cleaner in both cases, one guy retired, the other was unreliable.