Just to let you know that I have resolved the issue. So with the new BCM installed I checked if the light worked in the right hand side but this time it didn’t, so I tried the right hand cluster in the left and it worked. Weird especially as the left worked in the right with the old BCM. Therefore I was pretty sure now that cluster was defective. So I tried the right cluster in the left and it worked confirming my hypothesis. But still a bit puzzled I decided to compare the voltages on the outer cluster pins 2, 3 & 4.
left outer connector (old BCM)
pin 3 - 0.48v
pin 5 - 0v
pin 4 - 0.7v
pin 2 - 0.7v
Right outer connector
pin 3 - 0.48v
pin 5 - 0v
pin 4 - 0.48v
pin 4 - 0.48v
With the new BCM the voltages on the left where the same as the right 0.48 V which concluded that the BCM and light cluster were both defective.
On close inspection the left light has some water in it which probably caused the LED circuit to short out which induced a fault on the BCM. However I believe this was intermittent because it worked on the right hand side during my initial fault finding and that’s what made the whole fault finding process that much more complicated.
so I picked up a new cluster from a breaker and hey presto it’s now working.
This would have been a lot easier to troubleshoot if I knew what the voltages were supposed to be on the tail light connectors so I hope this may help someone else out one day.