Pretty sure i know the answer but cant find definitive yes/no.
Is there a way e.g. api call to make a page stored in google's amp cache to force it to get new copy of that page?
Editorial want to know when they update a page in cms that google gets updates too.
There are two ways to handle this: let Google do it and/or do it yourself. If you let Google do it, it's a black box - only Google knows how they manage the data they cache. If you are deploying data in your own CDN, well - no need to bore you with the obvious answer there.
To answer your specific question: no, there is no API to force a new copy of an AMP page in the Google CDN. However, if you use http://schema.org/dateModified in the AMP document and keep your sitemap up-to-date, you'll present the Google crawler with data it needs to accurately refresh its CDN.