Brand the 404 page
The appearance of the current 404 page is entirely off-brand! There's no Menu (why?), the page footer is entirely different, the colouring is different, and the font is different.
Also the wording Similar Jobs is inappropriate, as it's Latest Jobs that get shown and sometimes they have NO relevance whatsoever to the job in question.

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Andrew Gifford commented
Currently, our 404 page also displays a grainy low-res logo. Why isn't this the same resolution as elsewhere on the site?
The font is a pale colour, inconsistent with the rest of the website.
The 'Sign up for job alerts' isn't consistent with the 'Register' language - using two terms for the same thing.
And, currently, there's only one 'Similar' job displaying at the bottom of the message, where there should be three 'Latest jobs' (not 'Similar' as most aren't like-for-like).