adding 6 bulkhead lights

[dingbat]

Interesting about continuing through with cpc. I have a question. Do you need to do the same with 12v halogen spot lights ( SELV ) ?

you should NOT earth a SELV supply. the idea is that it is 'seperated'

No, but if you are looping from transformer to transformer with a cable incorporating a cpc you should maintain the continuity of that cpc even if it is not connected to the transformers themselves. If a cpc is present at an accessory it should have electrical continuity back to the origin of the circuit. Alternatively, use two-core with no cpc. (It's a bit like cars and indicators - if they're fitted they have to work... but they don't have to be fitted)