Poor Service from MiWay

My car went for repairs on the 9th Jan 2019 after being hit by an Aspen truck. The insurance got me a car hire from Avis for only 10days. By the 19th my car was not finished and they had to extend the hire to 25th Jan. Up to today my car is still not ready but they say I have to take the car back because they can't extend anymore which will leave me without transport to work for the next coming days. My asking them to extend failed. Why I don't understand is that what ever costs are inquired, they should claim from the third party but they refuse and went on to change the policy. Very unhappy with them.