I had a look around and spent an hour or 2 googleing rpm fluctuation / flutter but I can imagine people will describe it in such different ways that it makes searching difficult.
I asked my garage about the RPM issue and described it in detail:
Happening when warm OR cold.
Only happens under very light load.
Needle "bounces" around 300-400 RPM up and down
Usually only happens at low RPM (around 1000-2000), though I haven't driven more than 50mph yet to find out due to living in a city
Happens in all gears and in manual mode
The garage said the torque converter wouldn't be the first thing they would check. Also I have noticed it does kind of feel "unsmooth" when its doing it, but it doesn't feel like any significant deceleration or acceleration, but I assume thats what it is, and I guess the torque converter is smoothing that out considerable compared to the RPM change.
I ran a full scan of the car (as I cant scan my DME directly as it says the control unit is not the model it was expecting) and got the fault code 277F which I believe is crankshaft sensor speed.
The garage has advised this could result in unsmooth operation and that they would recommend replacing the sensor.
I don't know much about engine internals other than a basic knowledge of the theory behind how one works, does it sound plausible that that could be causing the issue?