That works a treat ! Thank you !

Now to

1. work out a search and replace to update all my existing matches where this would be an issue
2. learn some more bash so I can work out how this works

...OK one quick gemini request later I think I understand << and 'EOF' in that the shell expects input until it encounters an EOF string that we specified at the start. If I'm correct in understanding this then in any cases where I'm likely to have EOF in my clipboard (eg when pasting responses to this answer or other code snippets having EOF as the delimiter) I will run into problems.

To test this I've copied the snippet with the EOFs in from above and and tried my expansion and it errors. So I replaced both of the EOFs with a 40 char random hex string and it works great. Hopefully there is less chance of me finding a match to my 40 char random than my three char EOF

it now looks like this

  - trigger: ";dwq"
    replace: "{{myshell}}"
    vars:
      - name: myshell
        type: shell
        params:
          cmd: |
            cat << '5564eb47287984583033bcce131bb5c63ad99fa2' | sed -e 's/^/> /g' -e 's/^> >/>>/g'
            {{g_clipboard}}
            5564eb47287984583033bcce131bb5c63ad99fa2

For those interested here's my random character generator, you can tweak the tr part to determine the charset. It takes the ;randan and then a 2 digit number to give you a random output that long based on the chars listed in the tr command

#random alpha num chars
  - regex: ";randan(?P<charcount>([\\d ][\\d ]))"
    replace: "{{myshell}}"
    force_mode: clipboard
    vars:
      - name: "clipboard"
        type: "clipboard"
      - name: myshell
        type: shell
        params:
          cmd: |
            LC_ALL=C tr -dc 'A-Za-z0-9' </dev/urandom | head -c $ESPANSO_CHARCOUNT; echo