What do you want to do there?
Do NOT try to park on Sion Hill, you’ll be driving aronnd getting stressed. It’s really hard to find a spot there, as it’s all flats. Look at a satellite view to see what I mean.
If you want to go to the play area, the bridge, slide & camp areas, then park on Clifton Down (the road) opposite Percival Rd or nearby. There are loos near the bridge.
Otherwise, I suggest the Circular Road, near the edge of the gorge, close to where the other loos & ice-cream van are.
Edit: sorry – Bonchance gave exactly the same advice as me, but earlier. Which shows we are right. 🙂