The problem is that the original RTF Object is invalid while merge is called and hence these calls do not appear to be available. It was the absolute position in the text I was after not the screen position. But not to worry I have just finished coding a work around by creating an array with the start positions of all merge fields before calling MergeText and then adjusting subsequent start positions for the length of the text inserted - now working as I need it.
Thanks for your help.
Paul