The church is on the Northway Estate, just inside Oxford's northern ring-road
Get to the roundabouts at the bottom of Headley Way: From the northern part of Oxford's ring road (A40): between the roundabouts for the A40 (east to London/M40) and the A4165 (Banbury Road) you will find a turn-off for Marston and the John Radcliffe Hospital. Follow those signs. and you will reach a pair of roundabouts via Marsh Lane, with a BP Garage opposite. Turn left at the first roundabout, and go straight ahead at the second (there is a row of shops on your left). Now you are at the bottom of Headley Way.
Turn onto Northway Estate: turn into the road on the top side of the shops (this is on the left, if you are following the directions above); this is Copse Lane. Follow the road (it becomes Westlands Drive) past some more shops, towards a tower block. Before that block, turn left into Maltfield Road, then right into Sutton Road, where you will find the church.
You may find it easier to use an interactive map provided by Google.