@ecterrab 

Sorry for the delay in thanking you for this reply; I only had the chance to test this in the November 27 build of the Physics package. And yes, it works a treat now! I think this is a significant improvement, so thanks, and well done.

(Oh and yes, I'd seen that much of the functionality of what I was doing is built in; I am trying to understand how to "do" tensors, just in general.)

Cheers,

K.