Usual recourse is to spray with an appropriate insecticide, but obviously, the tree is too big to do that, so its not practical. You could try simply blasting it with the hose to try to wash off as many as possible.

You might try purchasing predatory insects such as lady birds/bugs. They will spread throughout the tree and devour aphids. I heard them to be very effective. Sometimes they just show up naturally. The tree actually sends out a stress signal.

I'm tempted with the hose idea Bamboo. Will have to look for ladybirds Bathgate, I'm in France so not sure where I'll get them.
Thank you both.

Many garden centers have them or you can order them online.

That may not be the case in France Bathgate...

I'd leave them unless the tree is suffering. Nature will take care of the situation in due course. All sorts of things eat aphids.

Good point Sid. That's kinda where I was going.

Thanks Sid, my local ghuru on wildlife has said the same. Just need hoards of ladybirds, hover flies, wasps and hornets now.

Haven't been on the site for a while, but just to update my tulip tree cleared perfectly with the help of ladybirds etc, I didn't have to do anything in the end!

Thank you for all your comments and suggestions.

That's interesting. Amazing how nature corrects itself in the end.