UC earnings less the £542 per month except for one month

I'm in the process of moving over to UC. I work 10 hrs per week. I started receiving the real living wage in January. Around then my tax code changed from S568T to S600T. My friend thinks I'm on the wrong tax code. I can only work 10hrs a week at a push. One month I was paid and extra £50 I cant work out of it was a one off thank you payment for sticking around from my work and there's no one around to ask. The online form asks the question "In the 12 months before you claimed Universal Credit, did you earn below £542 per month at any time?" I would have to be yes for every month except for this one month. However, it doesn't allow me to comment. I'd be grateful for any suggestions on how to proceed. Thanks