The 'download' buttons for layers in this data catalogue...
https://environment.data.gov.uk/ds/catalogue/index.jsp#/catalogue

...used to resolve to fixed URLs that we could call from FME to fetch the data.

They are now function calls on a widget...

<a ng-if="layer.format" target="_self" href="javascript: void(0);" ng-click="vm.performDownload(layer)" bs-tooltip="" data-title="Click to download">
<img src="images/download-small.png">
</a>

There are no requests when the button is clicked so a POST won't do the job of a button click.

Does anybody know a way to programmatically click the download button or to get hold of the payload by any means?