@TAO: Application of double jeopardy to s49
To my mind, at least, it's not clear that the double jeopardy principle would apply. The crime, for which he was imprisoned, was failing to comply with a s49 order. I agree with you that he cannot be tried twice for the breach of a s49 order- the double jeopardy principle.
However, there is nothing in ss49-51 of RIPA which prevent a law enforcement agency from issuing another s49 notice, seeking the same information - this is entirely different to charging someone again for the same crime. If he fails to provide the key, he is tried for the breach of the new order, and thus commits a new, triable, criminal offence. There is no double jeopardy issue here - it's breaching a separate s49 notice.
There are two competing policy issues here - one is that someone should not be tried twice for the same offence (although under attack in some situations), and the other is that someone should not be entitled to obstruct the investigation of a larger crime by committing a smaller crime, and take the penalty for that smaller crime as a way of preventing the investigation.
I'm not aware of any legal authority on this, so just going on the basis of what makes sense to me in terms of approach - I'd be very interested to see something which suggests a different approach.