Thanks David.
Yes, my original question to Zeiss was regarding the 'depth of field' at 70x zoom but I never actually found out what it was.
Having given the matter some thought and also now understanding how the Harpia is working internally, I can now use the focus much better. Still struggle at 70x at long distance but as you say, this is an expected limitation when you add together the movement of me panning, plus a bird in flight which is miles away and then also take into account atmospheric conditions. We were seawatching at around 7pm and could still see the 'shimmering' of the heat haze in the railings of a very distant ship.
One thing which I think set my brain off on the wrong track right from the very start was the way some people choose to describe the Harpia focusing. I read more than once that .. 'Turning the focus ring quickly, brings in the fast focus' .. which had me thinking exactly that, as though there is a sort of a 'centrifugal clutch' which kicks in when I turn the ring quickly and brings in the faster focus. With that in mind, I was turning the focus ring quickly in one direction and then as the focus started to sharpen I was continuing in the same direction but turning the ring more slowly thinking that I was now getting the fine focus. In reality this is still moving the fast focus part of the assembly but at a slower speed. I now realise (thanks to OhWeh) that the trick is to turn the ring in one direction and then as the focus sharpens, go a bit further and then reverse the direction you're turning and this gives you the fine focus part. I see now (and also feel) that the focus turning operation has short segment within the turning circle where the focus ring turns more freely and this is the fine focus part. It's as though the focus ring which is being turned has a small peg sticking out internally which travels in a short, 'empty' recess in the internal focusing gearing and as you're turning the ring, when the peg reaches either end of this recess and you continue to turn in the same direction, it then begins to push the fast focus part of the assembly around. It doesn't matter how quickly you are turning the focus ring, the fast focus comes in at either end of the small recess if you carry on turning in the same direction, the actual 'initiation' of the fast focus has nothing to do with how quickly the focus ring is turning and is simply a positional thing.
I hope that this makes sense and my description isn't too confusing.
Now that I have got my head around how the focus actually works, using the scope is much easier!