Hi Ramon, you were of course right 
By adding the “Set Relative Location” node in the Event Graph prior to adding the initial offset and then the animated offset, I avoid the strange “flying off into space” animation when exporting via the Movie Render Queue.
Here’s my setup for the Construction Script part of the BP actor (note that I now created a private variable for “Initial Offset”):
I then changed the Event Script part as follows:
By doing this then, I can animate with keyframes in Sequencer, using Construction Script to get things right and then tick the “Export Animation” option, whereupon the animation renders out exactly as it should:
You can’t imagine how long I’ve been trying to fix this problem 
So thanks a million for helping me solve this mystery!!!