There has been extensive roadworks on the stretch of dual carriageway that passes the park; it's about 8weeks since I last passed so I don't know if the works included access to the park from the GA direction.
If there is no new access then as Keith says you have to pass the entrance, go up and over the road incline as it goes over the railway keeping in the right lane, on the down slope towards the next set of lights (500m approx) take the right filter lane and wait at the lights until they change to let you cross the dual carriageway and then double back on yourself.
Turn right at the first lights after going back over the railway and then immediately left to run up to the park entrance and the parking area.
As Keith says check it out on street view first if you can.
Dave