Well, went out and picked up a second hand 7metre lance today, brought it home and had a play with it. The only pressure washer I had to hand was a cheapo Karcher 330 in the garage so I connected it up. This washer is only able to produce around 5L per minute at what pressure I don't know.
Anyway, the lance is horrible, heavy and very flexible. There was enough pressure to blow it backwards from the gutter. Emptying the gutter was pretty easy but terribly messy, blasting crud everywhere (got quite worried about next doors washing at one point). However, cleaning the soffits was extremely fast and fairly straight forward exept the windows were splattered in tap water (tds 300).
So, I conclude that jet washing is very fast, very messy and rather crude for clearing gutters, not as neat as a vac but handy for poking down a drainpipe. You don't need a mega expensive Honda driven water gulping pressure washer! I might buy a reliable electric one though, just for guttering type work - that's when my arms recover.
Mind you a ladder and a bucket is far simpler and much cheaper and easier to transport.