just pop along to your local PVCu supplier and explain to the guy behind the trade counter that your a window cleaner and a customer has asked if you could give her a price on a new unit, he will give you a tool to measure the thickness of the glass ( whilst still fitted ) and a few tools to take the beads out, no more than 25 quid
then offer it as a sideline
measure the thickness of the glass, measure the size of the PVCu opening , the opening that the beads sit in, then you can measure and order the double glazed unit without taking it out, then when the unit is ready, pick it up, take out old unit and fit new
its all very easy, if you want to do it in a serious way, i will, take some pics and explain in more detail in a bit of a guide
i know what i am on about as this was part of my job for a council in south wales, i headed a specialist PVCu team that did the work for the council ( the amount of courses i went on for this was frightening :
but a good laugh at the time )