I seem to be having a brain meltdown and cannot work out how to do a directional dissolve! By directional dissolve I mean where there is a certain area of the image that becomes visible earlier than the rest. I assume it must be done either with a solid and a feathered mask of some sort or using a black and white pattern and using a blending mode and opacity?
I have now pulled it off using a massively feathered animated mask on a black solid in combination with a fade down in opacity for the overall layer. Seems like it was just a question of subtlety with the feathering and opacity timing.