Partners car can only go into third gear on gears. Halfords stated £1800 for new clutch and gearbox however clutch seems fine just the third gear issue. 
 

no other gears work besides third. 
 

halfords stated the clutch and gearbox have gone and needs replacing however also been told could be a part from clutch or even gearbox.

need some help on this urgent topic. 

Halfords are just fitters.  They won't be able to repair a gearbox as that's a specialist job.

If you can't select any other gears with the engine OFF then it does sound like a gearbox fault.  It may be repairable if it's just the selector that's failed.

The choice is basically spending the money at a transmission specialist to have the gearbox repaired and warrantied.  Or taking a chance on having a good used gearbox fitted by a normal garage.

Either way, it's almost always worth fitting a new clutch while the gearbox is off.

As a 2019 model, I think this must be a Ka+.  It is not mechanically the same as the Ka which was only produced until around 2016.  This is important to note if sourcing a used gearbox.