Well, after playing around with the servo code the jitter is still there. It would appear that the Wire library used to interface with the i2c bus really messes with the timing that in turn causes the servo jitter. I happened to have lying around one of the Adafruit Servo PWM boards, these provide a hardware PWM controller controlled via a i2c connection – ideal.
Initial results look good although the costs are higher than using just arduino variants. However, a quick Google on our favourite auction site finds similar modules for just over £2 each. In addition, since the original plan was to use 8 servos per arduino, these modules support 16 servos each.