Victoria plum tree aphids?

If the tree is established and generally healthy, the aphids will be tolerable. They don't usually cause problems with the fruit. Unless the tree is being severely stunted by their attacks, I would not worry.

Soapy/washing-up-liquid water spraying is often used to get rid of aphids. It is quite possible that the "red/orange/black" are ladybird larvae arriving to eat the aphids, so you may not need to take action.
Aphids are worst in warm and dry years. The warmth allows them to grow quickly, while lack of rain means that they don't get washed off by heavy downpours.

Now that the leaves have curled and are aphid-damaged, the tree will probably drop some or all of the damaged leaves in a few weeks time, no matter what you do. It will probably grow more leaves somewhere else.
Young leaves are most favoued because they are soft. Aphids find it harder to suck sap out of older leaves that are thicker and tougher.

So....in summary....

If the mature tree has managed growth of new shoots of at least three inches (about 7.5cm), it should be strong enough to shrug off the aphids.
On very young trees, it can be advisable to get rid of aphids, so as not to stunt its growth.

Thanks! I think the tree has been there for quite a long time and there is no risk of it's growth being stunted as it's massive! I'm quite relieved by your answer as I was kind of wondering how on earth I was going to spray a whole big tree! I shall keep an eye on the orange/red/black things to see if they are munching the aphids.
Thanks for the help!