Make up a water fed pole, depending on high it is a harris pole would reach most house gutters.
Get a normal garden hose and plug it into their outside tap (if they have one)
Harris pole £15, Bentley brush from aldis £1.77, lenght of hose 6mm hose £5? A fitting, second one down would do
http://www.cleaningspot.co.uk/acatalog/microbore_fittings.html £2.45, couple of jets and a Y peice ... costs about £30 to make the pole up but once you got it, you got it, £30, and just plug it into their taps, could do all sorts with it, wash gutters, fascias conserv roofs etc etc.
Sound a bit confusing but it really isnt, ask me anything if you need to. And check out windowwashers video on youtube, hes a bit gay but hes ok really
http://www.youtube.com/watch?v=_Q8HhgHcz0M