Its worth paying to use Ospray Quay IMO. It shaves a good 30 mins of steaming off the trip out and is a cracking slip, it's not crazy expensive, loads of parking and space and a good pub / cafe (the Ship that Rocks) in the marina at the end of the day.
It's about 30 mins to the Shambles from Portland Harbour and very fishable in a small trailer boat.
Adam