@only4 We had a further look last night. Very interesting if you find that sort of thing interesting! The casting with the hole in that you pointed out screws onto the end of the quill. If you take the depth-stop off you can remove it completely. What it reveals is the underside of the lower spindle ball race, fortunately semi caged otherwise we'd have been cleaning and regreasing the bearing after finding all the balls that had dropped out!
The No1 Morse taper is part of the spindle, there's a collar on it that loads the internal face of the bearing but it's, as far as we could see, all one piece. Certainly it was going to get messy taking any more bits off.
It's a shame, the extra 100mm would have been useful, but it's a solid machine with no runout even at full extension, so for £150 I'm happy with it.