Oh right I see now. I was getting confused as when I watched a few videos, the aircraft has been engaged in VS and Speed mode throughout the latter stages of the descent and not in VNAV mode.
I suppsose playing around with the FMC during lower levels can be very distracting and is much more advisable to use the MCP.
How does the controller know what descent you are using - filed on the flight plan?
I tried various descents using the FMC on my flight sim addon and found it much easier to switch to the MCP below FL 80. Using the VNAV mode above FL 80 to fly the programmed descent was very accurate - impressive!
Many Thanks
Dan