CMS application within 1 year of variation order

Section 4(10)(aa) of the Child Support Act 1991 prevents an application to the child maintenance Service within one year of the date of a "maintenance order" in respect of the qualifying child(ren).

I have a Segal order, which was made more than one year ago but which was varied less than one year ago. My ex-wife has applied to the Child Maintenance Service, in my view prematurely. The CMS are saying that the 12-month rule applies only to the original maintenance order, not to any subsequent orders of variation. I am disputing this on the basis that:

1. The most recent order is as much a "maintenance order" as any previous order, given that it stipulates a new start date and a new amount of the periodical payments.

2. The legislation does not exclude maintenance orders that are a variation of a previous order.

The CMS seem very muddled on this, and few people there have even heard of the 12-month rule. Can anyone please confirm that I am correct and possibly point me in the direction of some supporting case law or guidelines?