Phil's Nestbox 2021

It's an interesting question as to whether or not adding another box to target blue/great tits actually increases your chance of getting a nest, and by how much. I did the same a few years ago, and by last year I had ended up with 5 blue/great tit boxes (28mm hole) and still no nest! What happened was that the local male did the rounds of all of them, and even brought his lady friend to a couple, but ended up nesting elsewhere (I am almost certain in the neighbour opposite's nest box). The problem is that bluetits are very territorial, and that territory might cover quite a few gardens. Your new box might be more appealing generally, so it might get used (as might the old one), but my feeling is that putting an extra box up certainly doesn't double your chances. Hence my pivot this year to try and tempt a few other species (sparrows and/or starlings), which still hasn't worked (yet!), but has resulted in loads of visits so far from both of those species. After seeing Pete's success with a robin this year, I'll work a bit harder on that as well next year (did put one up as an afterthought quite recently, but no robin..).
Having said all that, Paul does usually get multiple bluetit nests, but I believe that is usually a single male + multiple females.