Thanks for your reply. I will talk to the installer but I'm doubtful it will help. I've lost confidence in his plumbing skills since he's been working for me. I will ask him to at least cut the tile away and make the filters accessible though - if he can without doing any damage.
I also...
I have a thermostatic shower which is not getting past tepid temperature. It was fitted a couple of months ago and was OK to start with, but I think that might just be because I live in Madrid and the ambient temperature is much higher in the summer, so the cold water temperature was higher...