After many hours of thinking the blindingly obvious solution came to me at 4am this morning.

What was happening was actually a very slow continuous x rotation. Depending how long it was before looking after resetting the image appeared resized or inverted.

The tiny little index/rotate button in the hog profile had been inadvertently clicked on and recorded into the setup cues. The hog was showing a rotation of 0 on the output and I was so obsessed with trying to spot changes on the incoming dmx numbers that I didn't spot the x rotation on the HUD was showing 720. (isn't that supposed to be stop though?)