I followed the SW online turorial on How to Animate a Spring (approximate - circle swept and twisted around a "draggable" line axis).

The tutorial showed that you could mate the two end points of the axis to appropriate planes in your assembly, and moving the assembly would update the spring.  It further showed you how to animate by means of a motion study.

My assembly does not want to update the spring length - the spring part model has to be adjusted to control the assembly, it doesn't work the other way round and I get the error shown in the attached image. 

Had the spring part been a sub assembly I would have checked the flexible/rigid setting, but as it's not ... is there a similar setting that I am overlooking?

Yes, all other parts are set correctly; when the spring part is suppressed all is flexible.

Using SW 2012

Thanks and Merry Christmas