The pollen filter is meant to be changed at every service interval but it's only done as an additional customer expense and not included with the service. It really gets surprisingly clogged so reduces heater blower efficiency.
The panel is held on with plastic fir tree clips similar to this one. Give the panel a sharp tug and they should fall out.
.
Just in case anyone with a UK (right hand drive) vehicle struggles:
The filter cover is right at the very back of the footwell on the driver side, I looked at it blankly for ages because it's quite well tucked away. There is a little bolt that you need to remove first, and you could probably do with taking the overhead wirecover off (above the pedals), but I'm not sure its absolutely necessary (didn't seem to be too much in the way)