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Great, the good news is following submission of a sitemap via Webmaster Tools, things appear to be remedied on Google! It does seem, however, that the issue still persists on Bing/Yahoo. Some of the 404's are links from an old site that weren't carried over following my redesign; so that will be handled shortly as well. I've submitted the sitemap via Bing Webmaster Tools, as such I presume it's a similar matter of simply 'waiting on Bing'? Many thanks for your valuable insight!