When I've had employers try to claim weekend/evening work was "theirs", I've sent them a link to that legislation and a pro forma invoice for all my time during employment with them for the difference between 168 hours per week at equivalent hourly rate and what they've actually paid me so far. That nicely sets the tone for further discussion.
(I only got threatened with being fired on the spot once for doing that, on which they did not follow through, and I was already interviewing elsewhere anyway...)