Well tried both bays, same results, so either the EN-EL4a is faulty or Nikon are a shower of thieving ****s. Guess I'll just use the D700 with the EN-EL3 instead, its only a backup camera anyway, a shame really.
I do think it must be to do with voltage, and yeah, what is the difference between the MH-26 and the 'a' version??
Weird, oh well thanks guys.
This has partly to do with new laws in Japan regarding battery safety and output.