Hello Graeme,
It sounds as though you’re seeing a line across display when the display is always on, but not in apps or when using raise to wake. Apple Support Communities is a great place for help, and with the description you provided it doesn’t sound like a hardware issue. If you haven’t already try restarting your Apple Watch. You can find the steps for restarting in the following article.
How to restart your Apple Watch
If the issue continues after restarting test to see if the line appears when you have a different watch face set. You can find the steps for change the watch face in the following article.
Change the watch face on your Apple Watch
If you continue to have issues after those steps you'll want to reach out to Apple. You can find their contact information online via the Get Support website.
Take care!