Where do I stand?

Since my ex wife and I split it was agreed the children stayed with me 3 nights a week. This was fine until I started working longer hours (ironically so I could afford to pay her enough spousal maintenance) and didn''t get home until 7.30. She would ask the kids if they wanted dinner with her or with their stepmum- an impossible choice, and slowly but surely they dropped the 3rd night. As they are young teenagers I can''t force them. They are very happy here and get on well with their stepmum, but have fallen into a pattern now of going home that third night. The door is always open- on any night of the week, and they have holidays with us as well.
My question is- if the CMS calculations were involved that third night they now don''t always stay would cost me several hundred a month more to their mum. How would it stand? Given the original agreement is 3 nights and my door is open, and that they sometimes do stay that third night, would that still be classed as 3 or is it likely they will see it as 2?
We originally had an agreement that included spousal and child maintenance that is due to end because her circumstances have changed and I''m worried I''ll now be financially penalised in this area.

Under the Child Maintenance Service rules the paying parent must provide overnight care and stay with the children at the same address to be eligible for a reduction. If there is a dispute the CMS consider the terms of any agreement but may seek further evidence from the parties of the established pattern of care over the previous 12 months, or shorter period if the CMS think it is appropriate.