I finally got my backlit screens delivered from China (took 7 weeks) and ordered some ribbon adaptors (again from gooeysstore on ebay uk)
I used the instructions in this github gist. I have the 40pin motherboard, but didnt know which screen type I had (figured out it was the type A screen)
I ended up having to connect pin 103 to ground (I used the pin near the cart connector) It worked a treat, and I now have a backlit gameboy advance!!