Piddletrenthide straddles the B3143 which is reached from Puddletown on the A35. It is about 7 miles north of Dorchester. It can also be reached from the north via Sherborne on the A352. On a good day it takes about 2 hours from the M25.
There are direct trains from Waterloo to Dorchester (journey time about 2h 30m) and also to Sherborne (journey time about 2h 15m). Taxis from Dorchester South station should cost about £15.