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Have just made some changes to the site now, removed the SEO box - do you think this is better? Yes. I understand you may use that method in many other sites. I would suggest to you it is a mistake and with each of those sites you run a large risk of incurring this same penalty. It was done in a clear attempt to manipulate search engine rankings. I am confident 10 out of 10 Google employees who look at the page would feel the same way, and that Google's algorithms either can or should be able to detect such manipulation. You seem like a savvy guy and I have to think you agree on some level. The change you made is a great adjustment. It keeps the content on the page but positions it in such a way that it is not hidden at all. With respect to the URL removal tool, 404s are a natural part of the internet. If you encounter that situation again, my recommendation is to ensure your 404 page is current and helpful. There should be a search box along with your site's navigation. I would also recommend 301'ing any pages where a similar page would exist on your site. The 404 pages would naturally fall out of SERPs within 30 days in most cases. I can't say which of these two issues with your site led to your current ranking drop. I can share that I would classify both as major mistakes and you have done everything possible to resolve these issues at this point. If the hidden text was the issue, which is more likely, then check your SERP each day for when the cache updates. Presently your cache is from Aug 17th. When it updates, that is a confirmation Google has crawled the page and hopefully your rankings should return. PS. Your HTML sitemap is helpful for your footer. The XML and TXT sitemaps are not helpful and are unnecessary. I would recommend removing those links as a best practice, but they are not in any way related to your ranking issue.

Sorry for the late reply here. The help desk is the best place. That said, there was a known problem with UK rankings, but I think it's taken care of. Do contact them if you have any more trouble regarding this.