I think the root of your problem comes from a common misconception about the robots.txt file, Lewis.
A robots.txt no-index directive is NOT designed to get pages removed from the search index. It simply tells the crawler: "when you encounter this directive, don't crawl any further". So the crawler never even gets a chance to discover whether there are any further pages, never mind whether they might be in the index already
THEREFORE! Any pages that are already in the index will simply stay there. (And if any outside sources have links to internal pages behind a robots.txt no-index directive, those linked pages' URLS will often be added to the search index anyway!) Any pages which are in the index this way will have their meta-descriptions blocked from displaying by the robots.txt directive, as you are seeing in your case.
Since a robots.txt no-index directive stops the crawler from looking any deeper, the engines are blocked from actually discovering the 301 redirects on your dev pages, and so aren't getting the cue to drop them in favour of the new pages! Hence the dev site stays in the index and shows up in SERPs. The human user does get the redirect so ends up on the new page, but you still have the duplicate content/competition problem.
NOTE: to actually tell the search engines not only to not index the page, but to remove it if it already exists, you must add a meta-no-index tag in the header of individual pages. The robots.txt no-index MUST NOT be in place in order for this tag to be discovered and obeyed. There is an automatic setting in WordPress Settings -> Reading page to disallow crawling which automatically adds the meta-no-index tag to each page's header
Unfortunately, the problem is bigger than you stated, as I'm finding almost 70 pages from the dev site indexed in the.co.uk SERPs
Here are what I see as your two main options, along with their ramifications:
1. Remove the robots.txt no-index directive and allow the 301 redirects to be crawled, eventually causing the dev pages to drop out of the SERPS
- this would be the preferred option if the existing dev site pages have actually started to acquire incoming links and ranking value, but you'd have no control over how long it would take for the competing dev pages to drop out of the index, meaning they will continue to interfere with your SEO until that process completes
- you'll need to check whether any of the other 70 pages in the results have incoming links and if so 301 redirect them as well
- you'll need to add meta-robots no-index tags to the header of each of the remaining non-redirected pages on the dev site to get them removed from the index.
**2. ** Use the URL Removal Tool in Google and Bing Webmaster Tools to have the dev site removed from the index
- likely the fastest way to get the competing URLs out of the indexes, but would mean that any acquired link authority from the dev pages would be lost, not transferred to the live site.
- would still require either the robots.txt no-index directive to stay in place, or better yet, remove it and replace it with meta-no index tags in the header of every page on the dev site.
- you'd need to remove the 301 redirects
- since the search engines consider subdomains completely separate sites, you'd need to set up and verify the dev subdomain as a separate site in both Google and Bing webmaster tools in order for the URL Removal Tool to work.
I've never actually used the URL Removal tool on a full subdomain before, but see no reason why it wouldn't work as expected. You could actually test it out first on your dev.birdybanners.co.uk/ site as it has the same problem of the dev site being indexed in the SERPs.
Hope that helps give you a strategy to resolve the problem? Be sure to holler if you need me to better clarify anything.
Paul