Legally, I would guess that your remedy will exist in either contract law or consumer law since, on the face of it, you have paid for a service which has not been delivered as requested.
You might want to go down that road later, but I think in the first instance you would be best to contact your local authority who will ultimately be in charge of your graveyards and crematoriums. They should be able to help.
The plans are either wrong or the warden doesn't understand them, or worse still, perhaps your grandparents were buried in the wrong place. Either way, it is clearly unacceptable to have you place flowers on an empty plot. The council should be able to have the warden relocate the marker stone to where you know it should be.
If the council drag their feet contact your councillor and MP (or MSP). Court action should not be necessary.