Update:
I trained the model for 50 epochs and the resulting predictions were again blanks.
I then assembled a data set in which all the targets made up a large part of the images.
(small tagets potentially being the issue)
After training the new set for 10 epochs I got this result. Not perfect but not terrible.

However retraining the same set with the same settings from scratch, delivers this were everything is turned around:

Also training for more than 10 epochs again creates blank predictions.
Has anyone encountered this before?