Thanks so much for your reply. I have cleared all cache on W3TC and on Cloudflare but can see that when I load the front-end all the images are still being served as PNG and not WEBP even though they have all been generated.
I can also see the following under the Browser Cache section in Install > Rewrite Rules
location ~* ^(?<path>.+)\.(jpe?g|png|gif)$ {
if ( $http_accept !~* “webp|\*/\*” ) {
break;
}