Thanks all for your replies, the sail is new and apparently the newer ones are fuller than previous which should help with the extra kilos, however the sail / mast transition looks like it need smoothing out unless the mainsheet is block to block. I'm guesing but the flatness that you get from sheeting hard helps when the winds up and should help further flatten the sail if I increase the rotation. Likewise when its light to moderate and the sail is lightly sheeted and fuller the rotation still needs to be increased to correct the mast / sail transition.
Maybe I'll just attack it lightly and see what happens !

Thanks Phil