Thanks for the reply.
Ive tried various speeds and temps. The print in the image was printed at 190 deg c,0,2mm layer height at 75mm/s perimeter and 125mm infill (Kisslicer) I also tried Cura with the same temp layer height and 75mm/s, the print was slightly different but still had gaps the higher the print got. I have successfully printed with these settings before.
Its the same filament which I have had many successful prints with at similar temps layer heights and speeds.
I did add an additional fan recently but I've had quite a few successful prints with that working fine. I took it off anyway, it made no difference.
The extruder looks to be working fine, Its the version with the spring (bought November 2012)
There are no signs of slippage in the extruder when I examine the filament.
The Bowden tube appears clear.
What I don't understand is why does it only happen once the print is above 7mm in height ? All layers below this height are perfect. Could there be a problem with the Z motor ? I checked the height of the final print and its very close to the correct height.
Im very very confused!