Its a Kef cube1 connected to a Sony amp with a cable with a yellow end that goes into the "Subwoofer out" connection on the amp - the red light on the woofer turns green as usual but still doesn't work. Can anybody help with this?

Read the manual for the Sony amp. Learn how to use the test tones. Use the Test Tone feature and cycle it through until the onscreen display confirms that the amp is sending a test tone out to the sub. Make sure the biig volume dial is set to at least -30dB. Okay, at this point you'll either see that the sub is getting a test tone or there's no subwoofer in the list. If the latter is the case it's because the amp thinks or has been told there's no sub present. Read the manual about setting the amp so that a subwoofer IS present.

Now go to your sub. make sure it is switched on. If it has controls for volume then turn it up until you hear a rumbling sound.