A bit more progress this week. My extra parts arrived and I've assembled the light sensor (MKII). This one uses an Adafruit TSL2591 high dynamic range digital light sensor which, according to the specs can measure between 0.000118 to 88,000 Lux. It also breaks out IR, full spectrum and visible light readings.
The original LDR sensor I purchased was ok at distinguishing daylight from twilight, however it was not sensitive enough between twilight and full on darkness.
I'm also going to locate the new sensor remote to the rest of the project in a separate box and this has necessitated use of differential I2C bus extenders for that sensor. That part of the project is all wired up and I've tested the Arduino code to check it is all working (which it is). Don't look too closely at the soldering
Tomorrow I will be making openings in the two enclosures for the cat5 cable connectors and a bit of soldering to include the new sensor in the original project.