So we were very pleased to have a rental car, meaning that we were able to wander to many places that from the train or bus one might say "hey that looks interesting" but never get there.
We rented a car for the whole period we were in Soriano (24 days), picked it up at the airport and returned it there. This minimised daily costs to less than $Aust900 for the 24 days, with zero excess (from Economy Car Rentals). Excellent - a small Kia diesel that fitted into narrow streets and small parking spaces easily - and fuel costs very modest, less than Euros 100 in total.
Leaving the airport in Rome we headed north for Civitavecchia rather than going via the Rome ring road to the Autostrada del Sole. That is a more scenic route to Soriano, also the roads, on winter weekdays either way, were uncrowded and it was easy to get used to driving on the other side of the road without the heavy and more irritable traffic of the ring road.