Hi SimonClayton,
Thanks for coming to Apple Support Communities for help today with your Apple Watch not showing up as a data source in the Health app.
You should be able to see it as a source. Have a look here for some tips: Manage Health data on your iPhone, iPod touch, or Apple Watch. You may need to go into the Watch app on your iPhone. See the section under "Add information from your Apple Watch".
If it still doesn't show up, be sure both devices are fully updated.
You can also try unpairing and setting up the Watch again: Unpair and erase your Apple Watch.
Let us know further details if it's still an issue.
Have a fantastic day!