Thank you for your sympathetic words... it sounds kind of minor at one level, but totally blanking m when I spoke to her in a polite tone of voice flipped me into assertive mode!

Yes, you understood her nasty intent that was to get me banned for being aggressive! However the gym staff have known me for years - I'd intervened when one guy at the Reception desk was yelling at one of the older female managers (while the male fitness instructors did nothing!) so ... her victim-playing didn't have the desired effect!