[Closed] ARGH! Broken Saint rear mech - Spring gone? How?

Gutted, I have a new design Saint rear mech and the tension spring has slipped so the derailleur doesn't pull back and tension the chain.

I've disassembled the main body from the jockey wheel cage, re-set the spring but as soon as the jockey cage approaches a normal position the spring slips again and all tension is lost!

I'm not daft, technically competent, but unless some part has popped out I cannot see why it won't work again.

Anyone had a similar experience or a solution for me?

Cheers

James