How to unlock 'safety lock' on Indesit FM70PMR Oven?

The oven safely lock of my Indesit FM70PMR unexpectedly came on and the oven door locked itself and I couldn’t turn on the heat in the oven.
After some fiddling I can now open the oven door, but if I try to turn on the oven by setting the left dial, and then go to set the temperature on the right dial, the red safety lock light comes on and the oven won’t heat! Aaaagh!! Help ;)
Any suggestions re how to fix this would be gratefully received!