S300 main display light off when fully open

[lazaroid]

I'm trying to resurrect an old Samsung SGH-S300. It works perfectly apart from the light on the main display (can't remember about the small, "outside" display): as I open the flip, the light comes on with it partially open and then goes off when opened past about 20 degrees or so. If I hold it up to a light (so light comes through the outside display!), I can just about see the display and I can use the phone like this, i.e. it is switching on.

I've just been reading on here about how the backlight is controlled by a magnetic switch but I was under the impression that this controlled the actual display and the keypad too. I'm a bit suspicious that the keypad and display do work when fully open and it's just the back-light!

Could all this be explained by a magnet that's become unglued and slipped round, or something, or is it likely to be something else?