BT can trace withheld numbers that are being called from this country, they cant trace calls coming from overseas. right down the area that you dropped leaflets off and that will be a start for the police. also if they used a mobile phone the network operator can find out which mast it was coming from at that time. its called "pinging"
I would certainly report the offence to the police though, as if anything happens the police have the initial report to strengthen any case.