New pollen filter installed. The old one was filthy, so well worth doing. Also had a wipe around the area with disinfectant, so all nice and fresh now.
I found a wood chisel was perfect for levering the screw things out. Gently persuade sharp end between head and washer, then twist to lift it out. They both came out whole, and clipped back in. A bit of a sloppy fit, but not going to fall out.
The alleged pollen filter dashboard message was obviously misinformation from the other half. The new filter was in a sealed bag, so I tried fitting it first with the bag still on. Started engine, fan to full. No air out, no messages on display.
So the message was probably concerning the DPF. That makes sense, as I don't think we're driving long enough distances (or yobbo-like enough) to get it hot enough to clear it. Had a charge about in 4th gear yesterday to try and burn it up.