I phoned the emergency number on the park gate, and got a confused lady in Truro, who (after a long delay where I hung about while the hounds whinged about how it surely must be teatime by now) passed me to Cornwall Council Animal control, who (after another long delay when the hounds appeared to have given up hope of ever going home and went to sleep on the grass) passed me to the RSPCA, who said they would make it a priority case, leave it to them.
I was rather expecting them to call me back for more directions, but they didn't - however, the foal was gone this morning, and I'm pretty sure he wasn't in a state to walk out of there, so someone must have taken him. I hope he is OK.