Just because the radio and lights work doesn't mean the battery is any good mate especially when it comes to the business of cranking if it doesn't hold good charge and have enough voltage the plates will simply collapse and nothing will happen. I don't know whether ST's have the same function but leaving the battery disconnected for around 30 minutes does reset the ECU keep alive memory for the older MK1 focus' not sure on the MK2s. Can give it a try however.