Do these work for all types of Passenger Trains - from what I have observed to date it seems a bit hit and miss.

If I remember well, it is intended that the "Ready For Departure" lights does not always illuminate.

It depends on the type of train. In reality only a few trains are announced "ready for departure" by personel assigned for this job on the platforms

But honestly, I can not tell you for which this applies and for which not

ANyway, in my believe it is intended to work like this

Thanks - from my observations it appears to be working as you suspect.

It works, when the train has a clear cover signal until the main signal.

Noted - hence why Dispatchers might clear Cover Signal in advance of departure time.