A reference period is the stretch of time your employer averages your working hours over to check whether you’ve stayed within the legal 48-hour weekly limit. It’s normally 17 weeks, so a few heavy weeks can be balanced out by lighter ones later in the period.
It doesn’t mean every single week has to stay under 48 hours, only the average across the whole period.
You worked 55 hours one week covering for a colleague's absence, but only 41 hours the next. Averaged across your 17-week reference periodA reference period is the stretch of time your employer averages your working hours over to check whether you've stayed within the legal 48-hour weekly limit. It's normally 17 weeks, so a few heavy we..., you're still under the 48-hour limit, even though that one week looked like a breach on its own.