Done

It's probably hard to see the little blue light but it's there, just under the yellow and white rectangle with the green stripe, indicating that Bluetooth is powered and connected to my phone. Having failed to find a stable power supply inside the thing, the answer was to take a separate power source, in this case the scavenged power supply from an old Apple TV, tacked onto the mains outlet at the back, and use that. That's the yellow and white thing. The audio is soldered onto the Aux input. So now, when you switch the amp on, Bluetooth will start up and latch onto the last paired device, ready to play. Apart form being a little fussy about having a nice ripple-free power supply, it's a great little module.