Hello it,s the sprag (one way clutch) or the forward clutch drum has split,most common is the drum splitting.
To fix you either need to rebuild or replace the gearbox.
To change the gearbox you need to support the engine from under the bonnet across the inner wings,totally remove the subframe under the car and replace the gearbox.
Richard
rbtransmissions@aol.com
Richard
December 2006