@JamesD:
Sails:
I added Housemartin sails to my DF, an A rig and a B rig. I also bought the B rig kit, this means you can do a sail change in a couple of minutes, just swap everything above deck. I did not bother with a C rig - if the weather is that rough, I will be staying home....
The Mylar sails are better in light weather. The prime reason I changed sails was for identification, the DFs all look the same in a group!!
You can race in the DF65 restricted class with any compliant sails (not just the provided ones). You can make your own if you want. The DF65 rules clearly define the sails, specifying size and reinforcements. Sail material is undefined, but both sails must be the same type of material.
Batteries:
I sailed with the 4 x AA (rechargeable) for some time, they are fine but perhaps a bit heavy (125 gm) - I changed to a 700mAh LiFe battery and that provides plenty of endurance for more than 6 hours of sailing (weighs 37 gm) - this is under the minimum battery weight limit of 45 gm
Sail numbers:
The RG65 rules clearly specify the placement and numbering on sails.
Sail numbers for the UK - see link toward the bottom of this page:
http://www.rg65.org.uk/rules for how to get a number.
For other readers:
Sail numbers in Australia - find yourself the RG65aus Yahoo group. You need to get a Yahoo ID and then join the RG65aus Yahoo group. Check the database (under "files") and claim an available number. The group moderator will approve and update the database.
Sail numbers in other countries - no idea; sorry...