why has my iphone se compass stopped working

I think this was the route:-

Method 2 Calibrating the Compass

1. Open your iPhone or iPad's Settings. ...
2. Scroll down and tap Privacy. ...
3. Tap Location services. ...
4. Slide the “Location Services” switch to On. ...
5. Scroll down and tap System Services. ...
6. Slide the “Compass Calibration” switch to On. ...
7. Open the Compass app.

Easy when you know what you are looking for.

I talked to apple support, they fixed it. Something had turned off the automatic recalibration. Wasn't me, perhaps a software update bug. Working nicely now, the figure at the bottom of the screen showing the actual heading in degrees took a few minutes more to get working, but that came on soon after. Well done support team.