Tool change height cannot be changed - Bridgeport Interact 520 VMC

Hi, I have a Bridgeport interact 520 VMC with heidenhain TNC 155 control. I have various issues with the machine; Sticky limit switches, Buffer battery and parameters, dodgy Z axis encoder all of which have been overcome.
I have written some basic programs and all the axis are operational as is the spindle.
The problem I have now is the tool change height is incorrect. No matter what I change the parameter to the control indicates that the Z-axis has moved to 0.000 for the tool change. As it turns out I need the Z-axis to be at about -7.9mm to be at the correct height.
I have set the positive Z limit to be ~+7mm and the tool change height to be -7.9 but the machine always goes to 0.000 regardless.
Anybody experience a similar problem and know how to overcome it?

Many thanks,

WB