Sorry for the delay in replying, I only just came online and saw your question. Are you still looking for help with this?
If the grate over the drain is made of plastic, the normal remedy for this is to cut a hole in the grid and extend the waste pipe through it.
This way, you do not have large items like leaves etc, being blown down the grate.
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the grate is an old iron grating that is cemented in. does your reply mean larger food debris can be washed down gutter without blocking drains pipe leaving property as we have had issues with blockages caused by cooking fat and washing powder congealing.