Air bricks - Clay or Plastic

[daiking]

I need to fit air bricks in a new external leaf, 9" x 6" gaps have been left . As a new ground slab is in place there are 2 x 100mm diameter pipe in the slab leading through to the original under floor void.

Is there a preference for plastic or clay bricks?

 

(I should also check in the void that the ends of the pipes are open under there and no current infestations)

[Crofter]

Seeing as nobody else has chipped in on this one...

I am taking a bit of a stab in the dark but my preference would be clay, as I would expect less thermal expansion/contraction, no UV issues, etc.

[Nickfromwales]

And no nibble problems either   

Plastic does seem a strange option for down low, but I recall asking @JSHarris about his uPVC skirts covering his eps up stands and he stated that there was no issue whatsoever. 

If it's in brickwork then I think I'd go clay tbh, just for 'continuity'.