Hi James,
So far as I can see you have the following architecture:
- job posting: https://www.pkeducation.co.uk/job/post-name/
- jobs listing page: https://www.pkeducation.co.uk/jobs/
Since from the robots.txt the listing page pagination is blocked, the crawler can access only the first 15 job postings are available to crawl via a normal crawl.
I would say, you should remove the blocking from the robots.txt and focus on implementing a correct pagination. *which method you choose is your decision, but allow the crawler to access all of your job posts. Check https://yoast.com/pagination-seo-best-practices/
Another thing I would change is to make the job post title an anchor text for the job posting. (every single job is linked with "Find out more").
Also if possible, create a separate sitemap.xml for your job posts and submit it in Search Console, this way you can keep track of any anomaly with indexation.
Last, and not least, focus on the quality of your content (just as Matt proposed in the first answer).
Good luck!