the pot had one leg to ground, one leg to the amp input, one leg going to the - leg of the 4.7 capacitor.
Yes I think it is unstable breadboard stuff... I removed the pot and wired the mic output directly into the amp input, and altered the code a little so the motor driver doesn't over do it. This all works now and is nice and stable, so time to wire it all up properly now I think.
Many thanks for your help.