Aphids!
Try spraying with diluted washing up liquid, well a good squeeze in a watering can would do! This is emergency treatment!
Not in bright sunlight though, and then see what happens, you can buy a bug spray that is systemic,and you can rub away those you can see
Aphids give birth to live young(euww) at this time of year......loads of them.....so be vigilant

Thanks Pamg, I've googled aphids now, and I had thought aphid was another term for greenfly. Shows how much I know ! They are gross. I tried rubbing off the first branch, but the squishing made me queasy, so the wash up liquid it is. Can't really use spray as we have a lot of bees just now.

Any idea why they infest the Sambucus and none of the other plants (right beside a fuschia, a philadelphus, a bamboo, lupin and a large vibirnum bodnantense).?

You are right ,greenfly are green aphids or put it another way not all greenfly are green. :0)

They like nice soft stems to stick their feeding tube in and suck the sap, trouble is this can infect the plant with someting from where he fed earlier, and it weakens them too.

Mild soapy waterand if you can bear it with rubber gloves on, even a toothbrush, gently brush some off (not compulsory though)

Have you tried companion planting??
It really is the answer to these problems before they start.

GOOGLE COMPANION PLANTING;

Jet 'em off with a fine hose.