Saw some on ebay for around £100 and goes in the spare wheel space? 

Are these worth getting or is there another option I can use with factory sync3 head unit? 

Looking at something like the JX7T 19A067 AB

Will need to check if connection is in boot already as I don't fancy running new cables or messing with back of sync3.

On the Focus MK3/MK3.5 the original subwoofer is part of the 10-speaker Sony audio system. Compared to the 6 / 9-speaker audio systems the 10-speaker audio system has a different ACM module and different wiring. 

The 6 / 9-speaker ACM does not have a subwoofer output. If you want to connect an original subwoofer to the 6 / 9-speaker ACM you need to connect an aftermarket high/low signal converter to the rear speaker output and a low pass filter to filter the high tones from the signal. 

On a 6 / 9 speaker vehicle the subwoofer wiring will not be present.

 

Part number JX7T-19A067-AB is a B&O Focus MK4 subwoofer. This type of subwoofer has never been used on the Focus MK3/MK3.5.

I just picked up my mk3 facelift that has the sub in the wheel well. I can get the part number off it tomorrow if you need it. You would also need the raised floor supports as it takes a fair chunk of boot space compared to the normal mk3 boot floor level.

Thanks, dont fancy buying a new ACM, from what I read these were available from TitX+ 

I have Sony 9 Speaker although I disconnected the front centre as its meant to improve sound. Will see about the high/low signal converter