Check and see what kind of fields you've defined for Incidents::Svc_Ctr_ref and Incidents::Car_ref.
If either is of type calculation, set field will trigger this error as you can't modify the value of a calculation field.
If either has the "do not modify" field option set, you'll get this error message as well.
You've got the right relationships, but there are a number of different ways you can design your layouts and scripts in order to work with them as effectively as possible. You can see a few of the different possible approaches in this demo file: http://www.4shared.com/file/dZ0bjclw/ManyToManywDemoWExtras.html
There are also a number of methods you can explore for finding and selecting values (such as a service center or a car), that make searching large data sets more user friendly. Some of these methods are demonstrated in this demo file: http://www.4shared.com/file/plr_jbkk/EnhancedValueSelection.html
Thanks for the quick response.
1) I will check my field definitions,
2a) Thanks I will look at this file, and
2b) I will review this too