Singer Curvy Faulty on Delivery

On the 14 January 2015, my wife bought a Singer Curvy, when we got home we discovered that it was faulty and didn't work. We took it back the same day and after testing they agreed that it was faulty too. The lady then phoned the Singer SA and ordered a new Singer Curvy for us promising us they would replace the faulty one with the new one. The fault was in the meantime fixed by the local technician. (The faulty one was their demo model they had in the store) I was promised that they would still get me the new machine and replace the fixed demo model. This Saturday it will be nearly a month later and every time I phone to find out what is going on I am told by the lady that they are still awaiting stock. I refuse to work on the temporary demo model as it is unfair for me to work on this and then it gets sold to the next person. The machine comes with a cover yet I don't have one as the one they provided is totally to small for the machine. We want to know how long it takes to order and receive stock for this model Singer Curvy Model. My wife really wants to start working and has a lot of needle work that needs to be done. The local supplier: Laslap****and on the Westrand.