I'm guessing there is no easy way to invisibly mend a split in the knee.
The jeans haven't worn through anywhere else. They are Gap real straight and if Gap hadn't bloody changed the leg width and composition of that style I would go and replace them straight away. Looks like I'll have to start a (no doubt long and fruitless) search for another brand/cut of jean that suits me as well as my (now-ripped) jeans :(