One thought, and it may be a non starter, but is there any scope for talking to the uphill neighbours, where you feel at least some of the water is coming from, and asking if they could not in some way help? Either by addressing the lack of drainage on their land due to paving/plastic grass, or by contributing to the expenses you are incurring in part due to their choices, which might give you more scope. Or have you tried that?
@jenniferavril I remember one post recently when someone had taken out a Eucalyptus tree in his garden and realised it was a mistake because it seems that the Eucalyptus had taken quite a lot of water until then, and now, he has standing water. All comments agreed that Eucalyptus is a thirsty tree.
You've created a little (almost) water-proof trough, because there are no holes in the sides, and it's sitting on sticky impervious clay. You need to drill some holes in the sides, near the bottom, so water can trickle out. You don't want the holes to clog up so put a layer of permeable geotextile between the hole and the soil. Or you could lay a perforated pipe along the length of the planter, and make a hole for the pipe to poke through at the lowest end, so the water flows out onto the lawn.
"What is hateful to you, do not do to your neighbour".
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Just an idea.
I ♥ my garden.