For more general information on cookies see the Wikipedia article on HTTP Cookies.
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For more information on Google Analytics cookies, see the official Google Analytics page.
From time to time we test new features and make subtle changes to the way that the site is delivered. When we are still testing new features these cookies may be used to ensure that you receive a consistent experience whilst on the site whilst ensuring we understand which optimisations our users appreciate the most.
Hopefully that has clarified things for you and as was previously mentioned if there is something that you aren't sure whether you need or not it's usually safer to leave cookies enabled in case it does interact with one of the features you use on our site.