If you bought it in December in the UK, I doubt it will work.
The key differentiation is this:
Does it look like the picture above (one hole encompassing mute switch and volume buttons), or does it have two holes on the left side, one for the mute switch and one for the volume buttons like this one:
Image
One hole version should work fine, two hole version won't work with the 4S without trimming (which I wouldn't recommend).
Reason is the mute switch is moved ever so slightly closer to the volume buttons and the plastic between the two holes interferes with the switch by half a millimeter or so.