Thanks Sportster
I think may be starting to confuse myself

Anyhow had a quick look/check...
1) You should be able to measure battery voltage on the red output wire of the regulator while the engine is off. If not, the current which normally comes from the red wire of the regulator isn't going to push current into the battery due to no connection.
I disconnected both rectifier plugs and measured voltage at red wire, MPEM side (that is harness side), 12v, this is the bit that is confusing me, what is the output of the rectifier is via the MPEM permanently connected to the 12v rail.
It would be nice to know, in the MPEM, what's between 12v in (3-25) and 12v to/from Rectifier (2-15)
2) Also, these regulators wont operate if it's not grounded, make sure the ground of the regulator isn't open.
With both rectifier plugs disconnected measured resistance from black wire from MPEM side to chassis and negative battery terminal (earth) and get zero ohms.
Should the rectifier body/heat sink also be earthed? I assume not as it is painted?
you should be able to test 3 of the output rectifiers as diodes (flow current in one direction). The other three rectifying devices would be SCR's, which should measure open (not shorted).
DMM setup for diode test NEW rectifier:
Rectifier red wire connected to negative DMM lead, yellow wire connected to positive DMM lead - current flows, reverse leads nil current flow. Conduct test with rectifier black wire current flows one way...
Diode test black to red wires open one way other .927 v a little high?
DMM setup for diode test OLD rectifier:
Rectifier red wire connected to negative DMM lead, yellow wire connected to positive DMM lead - current flows, reverse leads nil current flow. Conduct test with rectifier black wire current does not flow either way...
Diode test black to red wires open both ways open.