???iI have a massive conservatory roof clean to do Saturday. One where the roof slopes down to the building. Gutter isn't wide enough to walk along so I'll use my vacpoles and feed them in from the side to suck the crud ņut. In the past I,ve shoved the vac hose along but the nozzle is pointing the wrong wayto get the best suction.
Still might end up crawling over the glass 
Thanks Dave, I thought that was the reason why you wanted a 90
To clear out box gullies/gutters like that I've found them very awkward to do access wise and most are very tight to get gutttervac tools in but still doable one way or another. Lead flashing and internal brackets getting in the way does my head in though.
Myself I use various silicone bends for the task 45, 90 either 51mm-51mm or 51mm to 38mm reducer and standard 135 degree 51mm with or without nozzles.
Regards to guttervac nozzles I discovered a great tool which is just using the handle part of a commercial wet/dry vac plastic 3 section floor tool I already have. It just pushes into a 51mm silicone bend and can be twisted around to adjust the angle you want to work at depending where you are working from. If the gap is much smaller then will try to use a crevice tool but is a right pain in the arse to use as is forever getting blocked.
At the end of the day after trying I would rather say I can't do the job than go clambering over a conservatory roof that's for sure.