The girls decided to take the secret writing a step further and see if other juices would have the same result.
In case you can't make it out we tried (in order) lemon, apple, lime, grapefruit, grape and pineapple.
The girls guessed it was the sugar burning before the rest of the paper but after some research it turned out the 'inks' weaken the fibres of the paper so the they discolour first. (the same article in About Chemistry suggested ironing your paper rather than heating over a fire, to prevent mishaps - now why didn't I think of that).
So one simple line from a story book has opened up a whole can of worms - turns out there are methods for using milk, onion, saliva and playing with acid and base reactions to hide your messages - something tells me we will be doing this for a long time yet : )