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That's exact;y the type of drain I unblocked last week. Had been blocked nearly solid with sediment and crap from years of neglect and probably from when we got our roof redone. I was able to get the cover off and used an attachment for my pressure washer (about £20 off ebay). Typical drain rods might not be flexible enough to get around a tight corner (the first set I bought were too stout). To be honest, even a fairly stiff garden hose would have worked for me given enough patience. The attachement I got I used to poke at the blockage to break it up and the pressurised water helped to flush it out of the way.
An overflowing drain that takes water from the roof gutters.
The grate is not moveable but there is a drain access metal cover about a metre from it.
I don't have the poles to poke about in there though.