Matt's Boxes 2019

I would personally give it until around the 15th / 20th April mark before you give up Matt,
Only saying this because when I have had more than 1 blue tit nest, Some have started mid April.
We refer to them as "Late nesters" when its getting on later in season and building is underway for most. 
Normally you get these if your male as more than 1 female (always been case for me) or if their nest as been disturbed elsewhere. 

Take 2016 into account for me
1st female - 21st March
2nd female - 16th April
3rd female - 21st April 

This was when my male blue tit had 3 females, This does happen sometimes! So I would give it until 20th, If your getting no visits etc by then yes but still hope for the next 3 weeks I'd say. 
Even if next week brings nothing.