Okay, so turns out I'm being a little impatient expecting Google to crawl my new sitemap so soon. I've just noticed a message in my Google WMT > Sitemaps dashboard saying "URLs ready for crawling".
Apologies for raising the alarm so soon, I had wrongly assumed when Google told me my sitemap was "processed", that they meant crawled. It turns out that processing comes just after submission, then crawling. Happy days.
Hopefully in a week or two - this should be updated. If not, I shall know to raise a support ticket again to let you know and see if anybody can help. For now though, it seems I'm in the clear.