Success.

On the 5th April I blipped the arrival of this nest box.  Assuming it was too late in the season to be of interest this year I didn't rush to put it out so about the 10th I stood it on top of an Ivy covered tree stump which we could see from the living room.  I didn't really consider this its permanent position.

About 3 days ago we saw a glimpse of a bird entering and once or twice over the last couple of days.  Last night we finally saw enough to see it was a Great Tit and that it was carrying a fecal sac.  So we had probably spotted them when they started their feeding trips.

A spot of mental arithmetic based round the smallest usual clutch size shows that in about 14 days they had found the cavity, examined it and decided that it was a des res built a nest and started to lay a clutch of eggs.  All without us noticing.

The woodcrete box certainly appears to have proved effective.