Finally had time to check the ageing issue out.
The plastic domes that come with these sensors are not UV stabilised:
After 3 years mine was chalky white and could easily be chipped with a fingernail.
I replaced it with a 25mm diameter glass half dome bought from a bead & crystal shop.
LUX reading went from 14,000 to 48,000 
Word of warning about the glass domes. They are cheaply made and often have a thicker “drip” of glass at the top of the dome. This can act like a lens and could easily burn the PCB. I sorted through 4 or 5 before I found a nice even one.
Now alI have to do is recalibrate all my lighting automations and my solar efficiency sensor.
The other thing I noticed was the non outdoor cat5 cable is going to need replacing or protecting. I’ll probably just wrap it in self amalgamating waterproofing tape (Scotch 23 or similar).