Ok so as I said guys don't let your mind over complicate your fault finding process, you always need to be methodical and start with the obvious.
The fault was reverse polarity on the supply tails, by switching the mcb off what I was actually doing was switching the neutral.
Easily diagnosed with a voltage meter
L-n gives supply voltage, but
L-cpc gives 0v, this could also be presented with a disconnected or incomplete cpc so to confirm reverse polarity ....
N-cpc will give supply voltage.