Cherry tree affliction ......

Runner beans
😁

You could spray with soapy water (diluted washing up liquid).

How do I get rid of black flies on my plants naturally?

Options include thyme, rosemary, clove oil and peppermint. Put a few drops of the oil in a spray bottle and fill the rest with water. Once sprayed, the oil will overpower and kill blackflies and their larvae. Be sure to cover the whole plant so more don't congregate on an untouched part.

There seem to be a lot of white fly around this year too.

Marigolds are supposed to deter black fly if planted around runner beams; I'm not sure if they work around trees.
They give off a scent which the flies dislike.

It is in a pot - a large one and I water it loads - it also has lots of cherries on it. I suppose all I can do is keep watering and hope for the best. Are these little critters specific to cherry trees? There is nothing on the plants in the neighbouring pots.

It will survive if it has enough water ie scattered water in a bigger diameter, not just a can at once down by the trunk. The insects are feeding on soft leaves and the leaves will strengthen if the tree has enough water. In time the preditors will come in and feed off the blak insects