Thanks for the response Dan.
It is worth mentioning that if I download the X200 manual PDF from Hitachi, that it is different to the printed manual that came with mine.
This caused some confusion for me until I realised it.
There must have been some revision or update of the X200 models.
The differnces that threw me were to do with register number changes.
As I said the Hitachi needs a value between 0 and 4000 in the frequency register.