PROBLEM SOLVED !!
Terrain can be made PASSABLE - also INVISIBLE.
The falling sphere is used just for 'something' to change and trigger the terrain action.
This is the new code - it may help someone (I hope!);-

#include <acknex.h>
#include <default.c>
#include "t_player.h"
//////////////////////////////

ENTITY* player="t_player";

ENTITY* terrain = "tester.hmp";

ENTITY* sphere = "ball.mdl";

function main()
{
level_load("xc1.WMB");

}


action down() //attached to sphere (drops to z-16 to make terrain passable and invisible)
{

while(sphere.z>-16)
{
sphere=me;
sphere.z -= 3*time_step;
wait(1);
}
{
wait(-2); //needed to stop 'judder'
terrain=me;
set(my,INVISIBLE); //needed to stop flash of light as //player drops through terrain
set(terrain,PASSABLE);
}}

Probably not very elegant, but it works. Got it by 'fiddling about' a lot - I felt it should work somehow.
David

NEW - FOUND A BETTER WAY!

remove:-

wait(-2); and set(my,INVISIBLE); and set(terrain,PASSABLE);

then replace with:-

ptr_remove(me);

Much cleaner and it works - hope it helps
David

PS. This is used for player who crosses terrain to tower in middle - enters tower - sets code and drops through to cave system below.