Yeah sorry guys, should have been a bit clearer! I'm using an external TLC549CP A/D converter which has:

ADC 'data out' pin attached to pin 0 on the pic.
ADC i/o clock pin attached to pin 1 on the pic and..
ADC CS(active) pin attached to pin 2...

Attached to the ADC is a potentiometer which is supplying the variable voltage (0-5V). Throughout my testing I haven't been adjusting it and so the serial output should remain the same if it was working correctly.

Haven't tried SHIFTIN/SHIFTOUT but will have a look into it... cheers!!