alisam:I do have something very, very similar to your link to the telescopic wand. I don't know where it came from (but was NZ online). It is a pretty robust telescopic wand which fits onto my Briggs and Stratton Power Washer.
So, all I need is to find an attachment and at least try it, before spending a lot of money.
Having an overflow on your gutter lower than the back edge might stop rainwater getting into the soffit but I don't think that would help with a high pressure jet that you couldn't see from ground level.