Disclaimer !

Try this at your peril !

BUT IT WORKS FOR ME

Get a good quality ring spanner or solid bar (not a ratchet handle) on crank bolt - make sure it if on bolt 100%

Put a piece of wood between spanner/bar and chassis rail to stop the spanner turning or put spanner/bar against floor

Disconnect coil - Remove spark plugs

Get a trusted friend to "flick" ignition key on and off

The torque a starter motor produces is usually enough to do the job

Ensure you have spanner/bar trapped the right way to undo as well

Watch fingers eyes etc for flying spanners though