I am afraid that this may be true, particularly if those responsible for the payments take a narrow view of the situation.
While the conditions of service for teachers specify a limit of 1,265 hours of directed time, with a mandatory midday break, heads, deputies and assistant heads have no limitation placed on their working time. Thus, while teachers giving up their lunch break to supervise may be paid for such work,under a separate contract, working a lunchtime is seen as a part of normal duties for senior staff.
This is the case, even though senior staff are "entitled to a break of reasonable length as near to the middle of each school day as is reasonably practicable".
It may be argued, of course, that this change was part of the price you paid when you accepted the move to the management spine and took the increase in salary which should have gone with it.
I have heard of one instance where a governing body took into account the loss of lunchtime payments in calculating the salary level of assistant heads, and I think they were quite right to do so.