Hi John
I don't know if your system is like mine but what you should be able to do is get the nozzle into the entry to the downpipe. Then listen to the vac. If the pipe is clear the vac tone will stay the same and the pipe should resonate, almost like a wind instrument.
If the pipe is blocked the tone of the vac will rise. You may be able to vac the blockage out but sometimes you can't. You will then need to then get the ladders out and dismantle to find the blockage or befriend a roofer for the very rare times that you will need him. I had one recently - huge weed in the downpipe right above the apex of the garage roof. No way I would have gone up but my roofer friend went up and sorted it.
Cost me £20 but the customer was delighted and that's what counts.