Your little black beetles are most likely to be pollen beetles, which
just love yellow flowers, and anything that is pale coloured or yellow.
They are particularly numerous at the moment with the oilseed rape
flowers being well out. Not predators, but will eat some of the pollen,
perhaps the aphids and bettles are just competeing. Once the pollen
beetles are finished on the field crops they will be in garden, but in
much smaller numbers until you hang out yellow or white clothes, then
you will find them!

Mike