Evidence of share care and variation expenses

Hi
Since separating a couple of years ago we have been following an informal family based agreement based on CMS child maint figures , discounted due to 1 night per week shared care. Ex no longer happy with that and wants to go down the statutory route (she believes she will get more money if she tells them I do not share care). I am not that happy with current arrangement (never mind paying more) as amount I currently pay is way above her form E figure for the child and way way above what she spends on our son, indeed I end up having to pay for some of his essentials (mobile phone, clothes etc,,,) because she spends most of the money on herself. In essence she treats child maintenance as spousal maintenance and is getting greedier (she received no spousal maintenance as part of the divorce as she earns nearly as much as me).

CMS rules on website seem to say that if there is no evidence to support shared care claims they use a default of 1 day per week – fine by me as that is what’s happening - but I’ve no solid way of proving that and probably end up being her word against mine (court order doesnt cover child access/maintenance). Does anyone know what happens in such cases ? e.g. what evidence CMS accept/expect. Also CMS talks about special expenses variation (i.e. reduction) to cover costs borne by non-resident parent - any idea what they allow ? e.g. cost that resident parent should cover but chooses not to? Or just my petrol money? I’m worried that system will allow her to lie about access, get more money and still leave me to fund essentials that she can afford better than I can.

Thanks