I have used them, pricey for what they are, the bracket is what you are paying for, if you want to do a good deep clean rather than just a wash you will need them for detailing and repair, and inspection of the roof, it again depends on the make up of the conservatory and the size.
i have been cleaning them as a specielist for ten years now and not needed them to often. as said you can hire them for £35 try to cut a deal for regular use and get the rpice you want for the job and add them on top, but in the price, but it really depends on what he customer wants for a clean.