Be quiet and suck up your 18% charge.
This summer I was working teaching on a summer course. Right at the end of the 4 week period my male coworker and I discovered that although we were doing exactly the same work, we had different job titles (his implied he was a manager, and mine didn’t) and he got £1 per hour than me. I had 8 years experience, and he was new to the job. As well as pissing me off, it made him feel crappy too.
(There is also zero point me complaining about it- it was short-term work and the head office was entirely staffed by totally clueless people in almost every other regard too.)