No, I think it was a user not reading enough documentation bug :-) It works fine if I put <br>s in the text. (Very much my first attempts with these sorts of scripts.)
On reading further, I also discovered that AirMail has introduced templates since I last looked, so I think there are at least three ways to approach this, which I'll put here in case anyone else is trying to do the same thing.
1 - use an Alfred snippet (just open a new email in your email package and type in the snippet). You could have one snippet for the subject, and another for the body.
2 - use a template in AirMail 3 (These can be called using keyboard shortcuts, as well as point and click, and can include attachments and subject lines and are very easy to make!). This is probably what I'll do. I couldn't find a way to generate an email using a Template from Alfred, although that would probably be my ideal as it would save a few keystrokes.
3 - create a workflow along the lines discussed. When I do this I get an extra line break at the start of the body of the email, perhaps associated with the sig file, but I've not investigated further. This would look a bit like this:
tell application "AirMail 3"
set myContent to "Hi<br>
<br>
Thanks for your interest in our amazing club... etc. etc.
" set newMail to make new outgoing message with properties {subject: "Joining our martial arts club", sender:"my.email@myemail.com", content: myContent} compose newMail activateend tell
Thanks very much for your help, deanishe.