So - this has now been fixed. Someone who does some contracting work saw my tweet about this and got in touch so I have him to thank!

Rather than just let this thread die without an answer though, I thought I'd share it.

So, within my web.config file I had the following security tag:

 <security>
  <requestFiltering allowDoubleEscaping="true">
    <!--allowDoubleEscaping needed to allow +++- in fund sheet file names-->
    <verbs allowUnlisted="true">
      <add verb="OPTIONS" allowed="false" />
    </verbs>
    <requestLimits maxUrl="500" maxQueryString="500" maxAllowedContentLength="52428800">
      <headerLimits>
        <add header="Content-type" sizeLimit="100" />
      </headerLimits>
    </requestLimits>
  </requestFiltering>
</security>

That in itself isn't an issue but when trying to hit the backoffice, the clientdependancy url is massive -

e.g.

https://dev.ourdomain.local/DependencyHandler.axd?s=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&t=Javascript&cdv=943896990

This exceeds the 500 character limit set in the web.config which then means the DependencyHandler can't load. I increased the size of the querystring allowed and tada! The backoffice loads.

Hopefully this might help future me or someone in the future that has a similar issue.