Many thanks , I've already seen this KB article.
We have SNI enabled on both core and branch steelheads already as part of our solution.
Without SSL proxy support, all webpages display fine, regardless if the page contains content from both internet sources.
WIth SSL proxy support enabled, those same pages are incomplete, and the formatting is wrong. Content is not making it to the client (looking in developer tools).
Again, each time it is tested, different content may be missing or the page may display correctly. No consistency.