Hi Lincfletch,
I understand you left a Family Sharing account and running into issues purchases applications. Did you delete any of the media that was purchased while using Family Sharing as these purchases aren't automatically deleted? Check to make sure the application in question isn't still on your iPhone, delete it if it is and download it again with your independent account. You will lose any data attached to the applications that needed to be deleted.
When you leave Family Sharing, your Apple ID is removed from the family group, and you stop sharing music, movies, TV shows, books, and apps with any remaining family members. In addition, any photos, calendars, and reminders shared by the family group will be removed from your devices. If you chose to share your location, you'll stop sharing your location with other family members, and your devices will be removed from the family Find My iPhone device list.
Any content that your family shared with you isn't automatically removed from your device. You can purchase it again or remove it to free up space on your device. If you downloaded an app from a family member’s purchase history and made In-App Purchases, you'll need to purchase the app yourself to access your In-App Purchases.
Any DRM-protected music, movies, TV shows, books, or apps that you previously downloaded from other family member's collections will no longer be usable, and other family members won't be able to use content downloaded from your collection.
You'll keep any purchases you initiated while part of the family group. However, if the organizer was paying for your iCloud storage or any other ongoing subscriptions, you'll need to start paying for them yourself using a valid payment method on your account. Learn how to add a credit or debit card to your iTunes Store account.
Leave Family Sharing
Delete music, movies, apps, books, and other content from your iPhone, iPad, or iPod touch
You may also want to try signing out of your Apple ID and back in again under Settings > iTunes and App Store.
Sign in with a different Apple ID on your iPhone, iPad, or iPod touch
Please let me know if these steps do not address this issue, and thank you for visiting Apple Support Communities.
Cheers.