Thanks to all who helped.
I have now solved the problem - after much (hours) fiddling about and trying everything I could think of - the name to call the player such that it will move along with a platform is
piBiped01_entity....
Could not find this anywhere in the manual or templates!
This might be useful to others with the same problem
Once again, thanks for your help.

so the code is:-
......
{
while(platform.x<300)
{
platform.x+=3*time_step;
piBiped01_entity.x+=3*time_step;
wait(1);
.....
David