I have two rs mark 2’s

a green 59 plate and a white 60 plate

the white one when running is around 14.5 then if you unplug the single wire plug off the alternator it DROPS voltage but the green one when running is 13.5 and when you unplugged the single wire plug it goes UP to 14.5 so it’s backwards!! Is there a reason for this. I have a brand new battery and alternator on this green rs and its luck if the thing starts every day!!!  The white rs has the same old battery and alternator and its starts 1st time every time. Anyone any ideas?

Green ones isn't charging correctly (as I think you've already guessed!). 

It's Smart Charge on Mk2's, not variable charging like newer models.  So it should be more or less 14.4v constantly - with the exception of a few minutes after a cold start, when it'll sit UP around 16v to pump as much charge into the battery as it can before the battery starts to warm up.  

Assuming the battery & alternator are correctly specced, the smart charge wiring would be the next thing to check.  I assume your temp sensors are all reading correctly on the dash?

9 hours ago, Tbs said:

the white one when running is around 14.5 then if you unplug the single wire plug off the alternator it DROPS voltage

That is correct. With the smart plug disconnected the voltage should drop to approx 13.8V depending upon the actual load on the alternator. This is the 'default' fail safe mode that will provide enough power to maintain a charge voltage to the battery without over charging.

9 hours ago, Tbs said:

the green one when running is 13.5 and when you unplugged the single wire plug it goes UP to 14.5

As Tom has already said this is wrong. I would suspect either the alternator is faulty or by some strange way the smart charge wiring in the harness/plug is crossed over. If both cars have identical alternators you can prove the alternators by swapping them over and see if the fault stays with the car or it moves with the alternator.