I think a photograph would be very useful if you could add one to your question, Howee...

That just looks like aphid infestation,blackfly, but that shouldn't cause the drooping, although the infestation seems to be rather heavy. Check the woody parts, the main stem and branches, for signs of scale insect - these look like little shields usually surrounded by white woolly stuff. Spray with an insecticide to treat the blackfly, if scale is present, that requires something else - you can wash that off with a weak solution of soap and water, or use a dry cloth lightly moistened with methylated spirits and rub them off with that.
I'm assuming this plant's in the ground, and in a sheltered, shadyish spot, and that the soil has not dried out (!). It might just be that the new growth has been nipped by cold weather prior to this heatwave.

Ok thanks - it was really windy a couple of weeks ago, might that have had an effect? There are two acers near it that are unaffected, is it likely to spread?

If the other acers are nearby and were also exposed to the wind but they've not got the same symptoms, it won't be the wind causing it.Although wind does damage Acers, they don't like it, so if that one would have caught more wind than the others, then yes, it could explain it.
If the damage has been caused by weather conditions, it won't be 'catching'. If it turns out to be phytophthera or dieback, then it might be.

I've checked for scale insect and cant see anything, and there are a lot of new leaves coming. Might it've just been severely munched by the blackfly? Can they strip a tree? It is quite small still

A very heavy infestation would cause the new shoots to die back - the aphids suck the sap, so if there are lots (and there are in the picture) there won't be enough (sap) left for the new leaves to thrive. What's peculiar, though, is that your other Acers have not been attacked in the same way by the aphids.

Ok thanks - could it be that the plant was already weakened and vulnerable?

Possibly, but only time will tell - you say there's new growth, so hopefully all will be well, but keep the aphids off this time.

Have had the same problem this year,only one of my many acers (with others in close proximity ) was infested by black fly causing the new growth to die back, a good going over with bug spray has worked well.

Ok, thanks. Is there an alternative solution that doesnt involve chemicals?

Well, Howee, in theory, you will be told you can make a weak solution of washing up liquid and water and spray with that - it should kill any aphids present, but will not prevent their returning as soon as the spray has dried. And I'd point out that washing up liquid is a chemical too, and not one aimed at use on plants... systemic fungicide will keep them off for up to 2 weeks. Up to you though.