Thank you all very much for the replies.
What i meant by the aircraft rolling when not on the centre line was that we were stationary, perpendicular to the runway, so say the runway is 360 degrees, then the taxiway leading to the runway was at 270 degrees. We then were given clearance for T/O which was staright away, (as only 4 flights land there a day), and we then taxied a little further then turned probably about 45 degrees to 315 degrees and then the pilot applied some thrust and then applied full thrust and then we drastically had to correct our position on the runway and we could easily feel and see that we were never anywhere near the centre line.
But anyway, thank you all for the replies
Best wishes
Oli