Inspect the leaves, stems and branches - you're looking for aphid infestation or scale infestation or any other critters. Add your answer beneath this when you've done that.

Thanks, I have had a look and on one side of the tree there certainly is some kind of infestation by tiny little black critters - the dead pods are absolutely covered in them, really thickly. Can I spray it with anything? It doesn't seem that the whole tree is affected.

Sounds like aphid infestation, and yes, you can spray, you need a pesticide which treats for aphids, if you can reach. In the meantime, try blasting them off with the hose - their excessive honeydew production causes stickiness and then dirt sticks to it, so hopefully, clearing the aphids will mean the black sticky stuff will go, assuming there's no other problem with the tree. Odd they're only one side...

Thanks,Bamboo,I'll try all that. The hose is a good idea as I probably can't reach to spray the top of the infestation, though I will try and see what happens. I did notice some ladybirds on the tree as well so may purchase some ladybird larvae to see if they will feast their way through the aphids. But I'd need a lot of them! Hope the tree won't die as a result of this attack. Thanks again.