Could someone tell me what this Symbiostock_XML_Cache is at all please? Is it needed for the site to function as I'm not including it in backups, but its a large folder.

http://www.microstockgroup.com/symbiostock-technical-support/tips-on-how-to-administer-a-production-symbiostock-website/msg341069/#msg341069

I can't answer what it's for, but (a) a cache is usually something created on the fly if it isn't there, and is thus by its nature only temporary stuff and (b) the above thread says you can exclude it from backups

When your site is displaying network results, these results are stored in symbiostock_xml_cache for future use. Search for something - if results are shown (almost) immediately, they probably were already in cache. If results are not in cache, site has to get them from your network friends, and it takes few seconds. Once got and displayed, results go to cache, so next search for the same item displays results much faster. Older files are deleted from cache once a day, so there are only results not older than number of days defined in Symbiostock->Network page (" days cache network search" under list of network sites).