I would definitely be using the scaffolding that is already up 100%.
Is the floor level enough for a tower scaffolding and it's quite high so can you get it close enough to the building to tie it to....or it will be rocking about up there.
Using the scaffolding that is already up, go spray the cladding at the top with virosol and clean as much of it as you can by hand or wfp, even clean all the window on the way down then just tidy them up again were required.
Price wise I would do all that for £500, it may be worth more but using the scaffolding I reckon I would do that in a day an a half max.