This was a comment from the OP, which effectively answers their own question:
After:
- Installing new Marlin 1.1.3;
- Changing the Maximum Acceleration on Z down to 20, and;
- Changing Acceleration and Retract Acceleration down to 1000.
I have now a printout that measures 24.65 mm (0.65 mm higher than it should) and good enough for me to say, this is OK.