It is entirely my own fault and I have no excuse.
Having planted it, I have stood by and done nothing for three years except screw a trellis to the wall ... and then another trellis ... and then another, as the thing went out of control.
Belatedly, I checked out YouTube and discovered it is supposed to be heavily pruned - and that, twice a year - to keep it under control and to make it flower.
Now, it is just a mass of leaves and tangled branches.
Can anyone suggest a way out of this mess ?