After some discussion with a CCTV Tech Guru I found that the LED night vision system was the problem.
Reflection of a "White Light" from any reflective surface, including the LED Screen can, and did, cause problems.
I covered the Camera LED's with tape (while leaving the Camera Lens clear), and the display digits are now visible on my 7" Screen.
Last night I use a simple LED Light Globe, still all going well.
Thanks Campbell for your thoughts, in questioning.
I shall continue using this site for other CCTV design hints.