Thought I was going round in circles here. Set html eg
<p><span style="font-family:Arial,Helvetica,sans-serif;"> to the Footer including both the Unsub token and the Domain Address token.
Preview mailing as HMTL in CiviMail. The Unsub text shows as intended, but the Domain Address shows as Times New Roman.
When I actually send a test mail, the domain address does show as sans serif. confusing.