Ah that's great. I didn't realise they weren't necessary if its externally vented - which it is. I assumed they were to prevent grease and particles getting into the internals of the fan/duct.
Thanks for the advice/help.
I had a new kitchen put in a couple of years ago but have only now discovered that the extractor fan, Howdens Lamona LAM2502, was installed without any carbon filters.
So is the fan/ducting likely to need complete replacement? I can't see the point sticking the filters on now that its had two...