Adding style to 404 topic

[WriterAndrew]

I have a docs portal based on TopNav.
I have a 404 page (index.html).
If someone follows a "dead" link, the 404 topic is displayed, but without any of styles or MasterPage layout applied.
All other pages appear with the relevant styling.
I've compared the 404 topic with a working topic in code view, and can see nothing obvious there,
I'm sure this is just a single parameter missing somewhere - but am at a loss as to which parameter, or where!
Any suggestions?
Many thanks

[WriterAndrew]

Just to add to my confusion - if I deliberately browse to the 404 file, then it is correctly formatted, but if it is called because there is a missing topic, then the formatting is missing!

[NorthEast]

If someone follows a "dead" link, the 404 topic is displayed, but without any of styles or MasterPage layout applied.

Styles could be missing if the link to the CSS file was broken, for example if you moved the topic to a different folder (after build).
The content added by MasterPage layout can't be missing though, as that's included in the topic's HTML at build time. So if the HTML content is different between the two topics that you're comparing, then they're different files.
So apologies if this is an obvious question - but you are you sure that you're comparing the same page (URL)?

Is the page included in the Flare project's Content folder?
Is the 404 redirect pointing to the correct URL? (presumably xxx/Content/Index.html)
When you view the page via a real 404 redirect, are there errors in the browser console (press F12)?