@... thanks for the multiple trigger suggestion, that's now working.

JJ Breen thanks for the liquid markup pointer, I've kind of got it working (but not quite), don't know if you can see what I've done wrong ?

"PRIVATE COMMENT": {% assign truncated_string = ticket.latest_comment | replace: '\n+', '\n' | truncate: 500, "" %}
{{ truncated_string }}{% if ticket.latest_comment.size > truncated_string.size %}{{ " - comment truncated" }}{% endif %}

What it supposed to do is replace multiple blank lines with just one, for those users that like to add surplus blank lines. This doesn't see to work, so either the string doesn't use \n or my code is wrong (very likely). Also after the string has been truncated (which does work) the second part is supposed to amend the string " - comment truncated." I don't seem to be able to get this to work - tried a number of different formats such as that above and without a {{ }} around the
" - comment truncated" string, but that string never appears in slack.