I managed to lose DS this afternoon at Verulamium Park in St Albans. After failing to find him I called the police, who searched the park and brought him back in a squad car
DS told me afterwards that he'd found a mum, like he'd been told, and that she helped him to look for me and then took him to the park warden, who handed him over to the police. Just in case she's a MNer, thankyou so much for looking after him
Thanks also to the family whose phone I used to call the police - sorry you were standing around for ages while they arrived! - and there was also a group of tweens/teens who helped in the search and gave DS a good talking to about staying with me in future 
4yo DD joyfully informed me that she's going to get lost next time so that she can have a ride in a police car!