No probs bud just keep safe if/when using ladders...
Maybe a drain jetter pressure washer hose might work but seems a lot of faffing around to me. I suppose it all depends if you can gain access to the bottom of the downpipe, what psi/flow rate pressure washer you use, whats causing the blockage in the first place as you still may need removing from the top. A telescopic pressure washer lance with a gutter clearing tool would work just as well I would have thought but whatever method you use you would need be carefull the pressure doesn't blow the downspout joints/bends apart.
If customers have an outside tap for downspouts that are too awkward to get at safely by ladder I would simply connect a garden hose & cone nozzle to a wfp like so in the pic below. At the time the pic was taken the nozzel was angled to flush through the gutters after clearing them. However I've found 2 plastic angle cranks work better if you want to point it straight down a downspout.
P.s if the don't have and oustide tap then you will need a pressure washer that can suck from a tank/water butt unless you can fit a garden hose onto a tap inside the house instead.
