Ex husband trying to reduce child maintenance - help

[Coincidenceschmoincidence]

On the basis he has the dcs 104-155 nights a year. Except he doesn't. It probably averages 80 but will end up at around 95 at the end of the year because we had our first 2 week holiday in 3 years earlier this year and he did a 19 night stretch over school hols. The days he does do are generally weekends, never days when I'm working or have to pay childcare, so financially they're no help.

He's sent a mail last weekend to say he was reducing payments (he's a high earner so it makes a big difference to us monthly) and will not respond to my (polite) reply. He's currently abroad so I can't call him. He works abroad a lot, hence him having the dcs a measly 3 nights this month

He's been fairly reasonable until now - he had been overpaying and I'm happy for it to reduce to what the csa guidelines say, but he's trying to argue new figures on the wrong grounds.

What can I do if he does just slash this?