Chev1 --- Legally as they have lent you a car there is not a great deal you can do, except to keep hounding them and being a nuiscance, until you get them to motivate themselves properly and show some interest.
If the car they have lent you is an inferior car, you could find out what the lease payments on that car actually would cost, then tell them you are only prepared to pay that lower amount, if that was the case, until your normal car is back on the road, that may pressure them into getting your car sorted quite a bit faster.