Right, rather than just pick up my net and have a quick look, I had a proper going over it last night as I had a "niggle".
Mike, are you sure you bought the GX? The rubber handle on mine is round (which was the bit I looked at), but the actual sliding bit of the net is a rounded off square with a channel. That channel and the slightly flat faces means that even when the locking mechanism is totally loose, the net handle cannot turn.
It also provides enough friction that with the adjustment set loose, the handle wont extend on its own, but a slight tug downwards before unshipping it from your back means it will extend and stay there. I had the fortune to put this into practice a few times last night and it worked perfect. A foot on the frame also works easily.
The net saga continues