Hi Mathew,
Well a couple of things stand out at first glance (not necessarily in order of importance)
1. You have a handful of exact / partial match links to both the home and secondary page, so potentially confusing to the search engines which one is best suited to show.
2. Some of the relevant links to the homepage are going to a .com version of the site which is being redirected to the .co.uk
3. All your internal linking is indicating that the internal page is the most relevant for this phrase.
All things considered I think your content/internal linking is the main issue. The internal page is much more kitchen focused (texts, products etc) while the homepage to me seems more focused on flooring. That in combination with the external links into the internal page and perhaps some lost value in the 301 redirects to the homepage is likely why the internal page is ranking for the term.
Beyond the technical side of it, as a user searching for diy kitchens I would prefer to see the internal page anyway, it is more immediately relevant so maybe a rethink of which phrases you are trying to focus for on each page is worthwhile and focusing the diy kitchen phrases on the internal page makes sense in this case.
My two cents!