Hi @woody567890,
I use Hardhat to verify, after first converting GitHub imports to npm imports.
I recommend trying the following: Verify smart contract inheriting from OpenZeppelin Contracts.
Using a single flat file means that licence and import information is removed and makes it harder to read.
When importing via GitHub, you should specify the release tag, otherwise you will get the latest code in the master branch. For OpenZeppelin Contracts you should only use code published in an official release. (see Importing from GitHub in the Remix documentation).
Importing from the master branch can cause issues with verifying as the code in the master branch can change over time.