Also you said "he might have to judge how much he cares about this" obviously he wouldn't want to lose his job over this but he's entitled to be paid the minimum wage. Are you suggesting that he should just accept being underpaid by about £400 a year rather than rock the boat?
Hi OP,
To clarify what i meant, in my original post i said that the link contained some things that your partner could do if the conversation with his employer didn't go well (i.e. meaning if they refused to increase his pay or pay backpay if appropriate). Some of the things at that link are more appropriate if you have secure employment at a medium or large employer.
You say that your partner works for a millionaire. I don't know the exact nature of the employment but if there is one employer making the decisions rather than this being a company or agency that he works through then it could be that his employment is relatively less secure than that of a person at a large organisation with a HR department. In these types of employment, doing things listed at that link (such as taking the employer to a tribunal, taking the employer to court or reporting them to HMRC) would inevitably cause a significant rift with the employer - I wouldn't want your partner to rush to threatening that type of action if it could cost him his job.
I did not mean to suggest that he should ignore this issue full stop.
Overall i think it's very likely that if you raise the issue as a probable accounting error, that will be sufficient to get things sorted.
The NLW changed on 1st Oct though
I don't think this is the case.
Does he get paid 4-weekly or monthly?
Usually for monthly contracts the earnings over a 52 week period are spread over 12 months, so this shouldn't be an issue.