Drivetrain F06 640d

Hi just had my 2015 640d F06 serviced at the local BMW. Was informed that it has few recalls, driver airbag and EGR coolant. It was suppose to be a day job then had call later that the car will not be ready as it also has another recall for the manifold. Prior to this my car going was running very quiet and smooth. It then took 3 days as the parts was not available. I picked the car up and noticed straight away that the sound of the car was very loud but assumed it may be due to the new manifold. Next morning whilst on the school run I got the following error on the screen “Drivetrain error - continue the journey at a moderate speed. Full performance not available. Have the problem checked by service”

so obviously I rang the local bmw dealers who carried out the work and was booked in for the next morning. Took the car in and spent 3.5 hours waiting to be informed that it’s nothing to do with the manifold. Been informed to change the exhaust back pressure sensor before turbine and the pipe. Also advised to change over valve for turbine control flap and wastegate valve faulty. Gave me a price for the parts of £286 + and labour £1280 +. Not accepting their fault on this. 
please can any advice me what to do?