Atco Balmoral 17S is failing to drive the metal roller?

I've got the pulley wheel cover off & can tell that this problem is not caused by a broken or slipping drive belt. Instead the large pulley wheel that is driven by the belt seems to be largely disengaged from the rear metal roller. If I push the mower along the ground (engine off), that pulley wheel does turn with the roller but is clearly not fully locked with it. The mower is around 8 yrs old.