Here's proof that if left unchanged - your cabin filter can ruin your heater matrix and your health. I waited way to long to change mine and what's manifested inside my dash is like something from a horror movie.
Here's how to swap out your old cabin / pollen filter in the Discovery 3/4 and finish the job with a decontaminating aerosol bomb from Forte.

LR170345 - POLLEN FILTER FOR RANGE ROVER SPORT 2005-2013 AND DISCOVERY 3 & 4 - under £5.00
https://www.lrparts.net/lr170345-poll...

DA6041P - FULL SERVICE KIT USING OEM BRANDED FILTERS FOR RANGE ROVER SPORT AND DISCOVERY 3 2.7 TDV6 - FROM 7A000001 CHASSIS NUMBER TO 2009
https://www.lrparts.net/da6041p-full-...

Chapters:
00:00 Intro
01:05 Removing the old cabin / pollen filter
02:42 Cleaning the glovebox
04:12 Using Forte Air Conditioning Treatment
06:08 Installing the new cabin filter