So, here we go, a summary of some of the research I have discovered so far
Parafoil kites seem popular for their simplicity of use. They pack away easily and come out of the pack instantly ready. No struts to pre-assemble. A large sled type seems to be the favoured one. In addition, a stability tail is needed, around 5m in length.
100m of line on a round spool is next. I'm still working out breaking strain but 50-80lbs seem popular choices. Some recommend Kevlar line for additional strength. Single line obviously makes deployment and control from Arwen much easier.
Attaching the camera 10m down the line from the kite is another popular approach. Attachment methods vary widely.
One seen involved a piece of small diameter plastic pipe about 30cm long, through which a a GoPro mount had been screwed. The kite string ran through the pipe and was twisted on at each end to prevent the pipe from sliding. String then ran from the upper end of the pipe diagonally down to either side of the attached Gopro. Complicated to explain but when you see it, simple.
Then there are the picavets made out of a range of materials ranging from old pieces of plastic meccano to highly engineered shuts from lightweight aluminium. Quite a few people have made the picavet cross base from 4mm dis plywood and then used simple lightweight screw in eyes instead of pulleys and this system seems to work just fine. Coat hanger seems to be the material of choice for attaching the picavet to the kite line.
Tough leather gloves are definitely a requirement, stressed by all kite flyers. Checking all knots twice, is prudent! Some people recommend making up a fishing line lanyard with a swivel clip one end. The lanyard is attached to the actual kite line and should the picavet fail, it will prevent the GoPro plummeting to earth! I hope! Some have a carabiner clip to store the picavet when it is u clipped from the kite line thus preventing tangles.
Two other home made gadgets I liked were the cleat on a piece of wood, an old tiller extension handle would do. This had a rope lanyard with carabiner clip. Basically the kite line is wound securely and tied off on the cleat. The cleat handle can then be attached around the Mizzen mast using the lanyard attachment.