Good news Sarah, still waiting for my roosting one to get a move on, my box still full of unmentionable stuff!!
I have recommended a visit to www.bto.org/blue tit nesting to another poster just this morning ... a greatly informative site for just about everything!
Fingers crossed for eggs soon, keep us informed on forum please, try 'Young of 2015' we do love BT's & pics from nest box cams!
Edit: Just checked my nest box as I could hear tapping sounds (is above my bedroom window) & it looks as if nesting has begun with a poo clear out, uploading a vid now although bright sunlight ring through entrance hole obscuring image!
2013 photos & vids here
eff37 on Flickr