Dear Dafydd,
We have an app that can link up all the variants to share the same stock holding. Basically it gives an option to share inventory among variant with a master inventory. It works best when your shop has different variations of products that consume one physical quantity.
You can install our app via this link: https://apps.shopify.com/accurate-inventory-app
Feel free to drop me an email (support@inscoder.com) and happy to discuss further. I can setup a test shop for you to try things out.
Regards, Billy