Right guys, you deserve a bit of an update on this.
I ordered the components as mentioned the other day - I am still waiting for them to drop through the door.
I do believe progress has been made however, as a friend from another forum suggested using a tone generator app using my pc's soundcard to drive 'through the rev range' and test the signal.
With this set up, the signal was as dirty as ever, and as i removed the signal source from my pcb, i noticed it was still seeing a voltage from the IC.
So wondering if it could be the cable to the pcb, i lopped it off a centimeter from the board and watched as the adc input dropped by half.
So having a peice of wire connected to the pcb can disturb my readings, even if the other end isn't connected to anything, and it only takes a short length to have a big impact.
So i think i'm almost definitely dealing with interference/noise - by putting all cables away from the pcb and hiding it in a drawer I was able to go from (8 bit reading) 50 (out of 255) which was jumping beteeen about 40 and 60 right down to 17 in the drawer which would on;y jump between 15 and 19.
That 17 then, kept as far from interference sources as possible, must be the noise footprint of my circuit.
So seems I need to look into working with noise.