Thanks for this, these are exactly the instructions I was hoping for.I pulled out some of the cheap fake leather covering the foam side of the headband and placed glue on the pleather. Using a plastic hotel key card, I put the pleather back into the slot formed by a scarily cheap piece of plastic and the main headband structure. I then squeezed the headband for about 30 seconds to set everything in place. I used only a modest amount of glue because I feared the glue would run off onto a visible peace of pleather when I squeezed down.
This is only a stop gap solution. After about 6 months it came detached again, so I had to glue it a second time. Hopefully this time it holds for a bit longer.
Best of luck.
I'll try my hand at gluing the cheap, fakely stitched, flimsy, styrofoam-filled, DIY-requiring pleather-plastic headband back together on these £1k headphones. Will post some pics of the inevitable mess that follows.