Cheers Steve!
This must be Steve Burke, right? After all, how many GT5-S Panteras are running around in Ireland? The De Tomaso world is a small world....
The thing to remember when filling the ZF is that there are numerous bulkheads within the case with relatively small orifices between each; if you just fill it up until it dribbles out the filler opening and then button it up, you will underfill it.
The thing to do is fill it until it dribbles, then walk away and let the oil inside settle. You will be surprised at how much more you can normally pump in.