Return to the main path by the park gate. Follow the path opposite the gate and under the trees towards the Observatory. Take the first turning to the right and walk a few paces along this path until you are clear of the trees. Then stop and face the Observatory. Here you can use a navigational technique used by sailors coming into harbour to walk exactly along the unmarked Meridian.
Look for the red line on a black background on a low building, in between the large dome to the left and the red time-ball on top of the redbrick building to the right. If you take a few paces along the path a white post (also on the Meridian) will align with the red line.
When the white post covers the red line you know you are standing directly on the Meridian. If the white post is to the left of the red line, you will need to move to your right (west). If it is to the right of the red line, you will need to move to your left (east). Keeping these markers aligned walk towards the Observatory across the grass until you reach the next path. Follow this path up the hill to the Observatory itself.