Overnight camping is allowed here provided you pre-book. Park in the overflow field of official Mill Road Car Park opposite entrance to Arundel Castle. Gates are locked between 6pm and 8am. No entry or exit during that time. Toilets in main car park but NOT available (locked) between 6pm - 8am? Town p within walking distance.
It’s £10 per night and that is collected my a very friendly man who’ll give you a ticket for your window. He’s about during the day so if you arrive early try and seek him out.