I have recently obtained job, which will also involve cleaning one window @ 75ft I would like advice on best pole for job. my other work does not exceed forty feet ,so if I buy a pole I would wish it to be adaptible for this work as well. All advice gratefully accepted Many thanks Kieron 
Presumably there are other windows below it and on the rest of the site. What many miss is that doing that one window gets you the whole contract.
Ok well best I can do without pics etc. Do the 75ft on the job last (but after you have had experience) and work downwards to the ones below on that section. I assemble mine by holding in one hand at 5ft and fitting the next pole into the base. But at that height "plate balancing" skills are required. So For the high stuff take an assistant to steady the pole while assembling, he would have to stand on a box or be quite tall and hold the pole in both hands inbetween you fitting the extension sections and also hand you each section, Its a two man thing. I can do it by myself but I suggest you use the extra man. If you dont have a roller brush use the back of the elbow to scrape the pole upwards then turn the brush over to work.
You will find that once up there the brush angles change as the pole becomes parallel with the wall which can be a problem so you may also find adjusting the adjustable elbow before you assemble better. You need a brush with good splay,which you angle the bristles slightly upwards and allow the upward movement of the pole to scrub and the downward one to rinse.
Once you get it up there Use the 2nd man to stand under the pole and help stop slide slip and assist in pushing the top of the pole outwards a bit.. Then drop off a section or two and do the windows below.
I agree with Tony, you are in for the fright of your life

it can be scary, take all safety precautions.
Its not exactly the way I would do it but its my best shot to suit you.