No torque on the Flymo cutting the grass so 5 mins on Google suggests the belt has snapped or gotten really loose.

Stripped down to find this:

You can see the pulley guides on the motor end are fine but it’s smooth on the blade end, I presume that is supposed to be grooved like the other the motor end?

Hard to complain as it must be 30 years old?

Nah it just grips, not that, belt might have been slipping though, degrease the wheel/new belt maybe? or get a new one and marvel at how enjoyable it is to cut the grass in the 21st century!