I worked on a client site where the expected week was 40 hours, 8 hours per week.
The client paid my company every week for 40 hours of my work.
But the company paid me for 38 hours, no overtime or RDO.
Is this legal under employment law?
This is a common issue in IT/professional services. I assume that your employment contract had a "you will be required to work 38 hours per work or hours as reasonably required to complete work as instructed" (or similar) clause?