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I'm no platypezid expert, but my tip is to find a sheltered glade with broad leaves (sycamore is great) and lots of honeydew - you could try spraying sugar solution on leaves if the ladybirds have eaten all the aphids. The platypezids will run around on the leaves imbibing honeydew in the sun. (What a life!) They can be swept, but I find I have more success targeting them individually.
Also look for large fungi which may be attracting adults, or may be filled with larvae - they are quite distinctive and can be fairly easily reared.