After turning off the A350 into Sturminster Marshall, the Old School Hall is about half a mile into the village in Church Street, next door to (right of) the Red Lion pub which is directly opposite the church. Drive into the pub car park and turn right. The entrance to the hall is at the back. There is plenty of space to park.
If you come in on Station Road, this becomes High Street after the small roundabout and becomes Church Street further down.
If you come in on Newton Road (near the Black Horse pub), turn left at the first T junction and then you are then in Church Street and almost there.