Dear customer, first i want to apologize for my long delay to get back to you. i had a personal issue and i am rreturning now.
I understand the query you are making in reality, almost 1800 pounds for the repair is not out of the ordinary because this repair is effectively including the CAS and DME modules in addition to the labor, installation, programming, etc.As long as the parts are new, they have the last software for your BMW, it is within reason.
Now, it is true that you can save a few pounds by getting second-hand parts as what you are doing the only thing you have to take into account when choosing the parts to pay for is that the serial models match basically when you go to buy the parts, corroborate the serial number of the vin code of your BMW 2006, it should match the code of the modules to be installed.
In addition, these modules must be programmed with the vehicle and this has a cost, this programming is not complicated to do, but it requires a diagnostic tool with its BMW software. In addition, the technician must have the knowledge of the use of this diagnosis but you can bring them the modules and they can install and make the relevant programming. I think that in total with the labor the programming and the installation you could save between 500-800 pounds according to the prices of those who get the modules and the prices that technicians charge you for installation...
About a good garage to do it:
29 Neville Road, Croydon, CR0 2DS
They are working and can give you a quoete for free.