Great, thank you old! I'll post back when i view the mower/beltI have had grass build up in the pulley on the crankshaft causing belt to slip especially when grass is damp or belt gets wet so would check for that if belt is slipping.
Another thing would be engagement cable adjustment. Does it pull the belt tight.
Does belt look shiny and smooth on V edges. If yes this is also a sign of belt getting older worn and possibly slipping a little.
If belt does not slip and drive does not work would point towards a problem in the gear box. No idea on if gearbox is repairable or replacement type item.