Spoke to the people at the Weymouth and Portland National Sailing Academy and they sent me the following info.
The cost will be £24.20 day membership, per boat per day + £4.70 harbour fees per day (although a weekly harbour fee of £12.60 will be cheaper). If you would like to moor the boats on our pontoons overnight, the total charge per boat will be 3 x day memberships + harbour fees. Whereas if you will be mooring in Portland Marina, the cost will be 2 x day membership (1 x day to launch, 1 x day to recover).
Plan
I will plan to leave the boat on it's trailer on the Friday 17th within the academy secure area. Access code for the gates will be given to me for arriving after hours. Cost for Friday nothing.
Slip the boat and have a days sailing on the Saturday, charges £24.20 plus £4.70, leave the boat on one of their pontoons.
Sail Sunday morning until mid afternoon, slip the boat out and travel back, cost £24.20 plus £4.70.
Other options are to leave the boat in the marina (separate arrangements needed with marina) although you still pay the academy for days membership (includes slip fee) for the day you put the boat in and for the day you slip the boat out.
Saturday night drink and meal will be organised if enough people interested
Peter