50-50 custody @ child support

Hello.
I divorced from my ex-wife five years ago.
From the start we agreed that we would have 50-50 custody of our son, who is now 7.
We operate on a two-week cycle where we each get 50% of the school week and alternate weekends with him.
We settled all finances amiably and I agreed to pay child maintenance.
I currently pay her ~£350 per month. This is a fixed % of my gross salary.
In addition, we split all child-related costs proportionate to our incomes, which covers school bills, additional childcare, uniform, his birthday parties, etc.

The situation changed recently.
Previously I was the higher earner by over x2, she had no savings/pension and no assets. This is why I agreed to the maintenance as she would need it to sustain our son's lifestyle. However that situation has now changed and she has inherited a large amount of cash and assets, in addition to getting a higher salary than she did before.

So my question is two-fold:
1. If a couple shares custody 50-50, how would that impact child maintenance amounts? What would a court say if a couple did not agree on maintenance amounts? (this is out of interest, I'm not going to court!)
2. As her circumstances have now changed, would it be unreasonable of me to request we alter the maintenance? My logic is that, if we have equal assets and wealth (she probably now has more than I do), she also has a good salary well above UK average, and if we look after and invest in our son equally, is she entitled to child maintenance at all?
Appreciate your thoughts and advice!