I actually got the circuit wired up again and, somehow, it seemed to work this time. I saw a voltage change with small taps and large hits so the next step was to connect the ADC (MCP3008) and the Pi Cobbler (GPIO cable connector).
So, I consulted the project at http://learn.adafruit.com/reading-a-analog-in-and-controlling-audio-volume-with-the-raspberry-pi/overview and wired up the GPIO pins to the ADC per their instructions.
I also took the python script from the same project and hacked it to remove the audio volume stuff.
It was during my initial test that I found I had connected the GPIO cable the wrong way round on the Pi which meant that nothing worked. My first thought was that I’d broken something but I pulled everything out and did a quick LED test ( see https://projects.drogon.net/raspberry-pi/gpio-examples/tux-crossing/gpio-examples-1-a-single-led/ ) which proved that none of the GPIO pins were responding as they should have done. So, I took the case apart, turned the cable around and reassembled everything. This time, the LED test worked as I expected (cue a quick Gangnam across the living room to celebrate ).
After wiring the ADC up again, it was a success as this video shows:
Next to do is to remove the capcitor to see if I can get the slope to decay much quicker.