Seriously though how hard can it be ?
Step 1 check your fruit doesn't have any dodgy enzymes which interfere with Gelatine. (That's kiwi,pineapple,papaya, and figs as far as I know).
Step 2 Juice your fruit
Step 3 Make the juice upto 500ml. This is where you adjust the sweetness add booze and otherwise adulterate the juice to provide a better jelly.
Step 4 Make a Gelatine solution
Step 5 Set
Step 6 EAT
Nobody seemed to think watermelon and gelatine presented any problems, so it was juicing time, slice the melon up (catching any juice from this stage) and pulp in a blender, pass through a sieve and away you go.
Step 3 had me worried, melon is a delicate flavour and I didn't want to wreck it, so I added a dash of white rum, a spoonful of sugar and made it up with water. At this point I remembered something from the Fat Duck Cookbook about adding malic acid, so I added 1/2 a teaspoon.
Hours later and the jellies smelt fantastic, time to unmould & taste.
Fortunately they tasted as good as they smelt. I'll be adding watermelon to my list