Files containing stenographically encrypted data are not undectable, the technique has been around for some time, and agencies involved in cryptoanalysis certainly know what to look for, and the resources to crack them. But its an entirely different proposition if you are accused of having such files, and you have to prove that they don't contain hidden data.
The advantage of stenography was that there is plausable deniability for anyone handling the seemingly innoculous files, unless were caught red handed de-crypting them. However this law is so vague and wide reaching, that you can be procescuted for having data in any form which can't be shown to be uncyrpted or rendered in plain text on request. And under RIPA as the fact that you have read this article may be stored for up to 7 years and used against you, you can't even deny that you are aware of techniques such as stenography.