Lovely shots. :thumbs:
In my limited experience, I'd say that filters can often add that grainy film over the top of your images, particularly if you zoom in heavily on the RAW. I've found that even the higher end filters can do this - Hoya for example.
What type of filters James? Polariser? ND? I don't think they should add 'grain' per se, but if you have slightly underexposed as a result of using a filter and you try to fight the effect in your editing software, you are going to amplify the background noise - which is always there to some extent, just waiting on you allowing it out to play.
I've banged on a few times about Exposing To The Right (ETTR). Doing this is a great way to keep noise at bay when you are tweaking your images. I'm now shooting most of my stuff at either +0.5 or +1 EV (though that is in part due to Canon's tendency to underexpose shots).