Well that was a dirty filter, lots of pollen and stuff trapped in the pre-filter (the pre-filter does a good job).
I cleaned the media (old tank water) and all is now good.
I am considering cleaning the other filter (filter 1) again next week just to get rid of all the pollen.
I have bits of debris now in the water column so I'll let that settle over night before attempting to get the camera out.
I actually enjoy all aspects of aquarium and filter maintenance, even when in pain.