Open-fronted nest-boxes

I have to generally agree with Paddy, depending on location and how managed they can be rather too open to predation. Spotted flycatchers etc will often shy away from using them if there are better (natural) alternatives. However, I have seen an open-fronted box used successfully but only when well established creepers were covering most of the box so that the entrance was reasonably well hidden.

So my suggestion is that if you don't have sufficient natural crevices for spotted flycs etc to use, such as only having a tall well-pointed featureless wall or a tree without natural cavities, then by allowing ivy, clematis or some such creeper to mostly cover the box, you may create the kind of crevice these birds are actually seeking. I have seen spotted flycs breed successfully in an overgrown walled garden in a rather aged, creeper-covered box. This was a long time ago (and in the Isle of Man!) but it worked.

For you're nest-boxes erected anyway, please also have a look at the BTO nestbox challenge as with that number of tit boxes occupied, it would be very worthwhile registering these and reporting the outcomes to this national scheme.