I managed to get it working. I was trying to use text to store date-related entries with most recent first. So I did ...
STDTEXT.GetHeading (to see if any already existed)
STDTEXT.GetText (to read all existing text and temporaily hold it)
STDTEXT.SetText (to load new lines, and reload temporarily held existing lines)
The tricky bit is that the text (attribute 'TextLine') is one big string which is comprised of 20 lines, each 160 chars long, so you need to manipulate appropriately. ie. Lots of the text in Ellipse is 60 chars long (so the lines are padded with 100+ blanks). Some other text in Ellipse is 80 chars long. You can check the allowable length of text per text type and setting are held on the "XC" type on MSF010.
From memory, I recall that the STDTEXT.SetText overwrites any text which may already exist. I think there is another method STDTEXT.Append which can probably be used to tack new text on the end of existing text.