Hi, and welcome @ollymac1!
I’m glad you were able to verify your domain successfully! I’ll also share this post: Custom domain verification for anyone reading this and hitting the same issue. 
How can I enable Vercel DNS when this tab/option is missing?
That is strange! I can ask internally, but can you can try to:
- Remove the domain from your project and add it again.
- Check again if there’s an option to switch to Vercel DNS in the domain settings of your project.
- Ensure that you’ve fully completed the domain verification process.