Water Inside uPVC Window Frame

I've just replaced some worn and broken friction hinges on one of my uPVC window units. When I removed the screws retaining the hinge to the bottom of the frame there followed a rather fine fountain of water from the screw hole! Simply by the height of the fountain Im guessing the frame was 1/4 full of water. I doubt I'll find the cause of the water entry and therefore cannot stop it getting in but I cannot see anything at the bottom of the frame to drain any water. Would it be ok to drill a small hole right at the bottom of the frame, just above the sill, the let the water drain?

 

For a moment there I thought that was a stroke of genious... well, it is and I can see how that could happen but sadly it's not the case. It did however prompt me to invesigate further. There are drain holes drilled and they do drain - I have tested it. But there is a partition just behind the drain that separates the sections:



The line shows where the partition runs and there is no means of draining the partition that has the hinge screws. I can fix this by carefully drilling into the partition to join the two from the existing hole but I can't help feeling I shouldn't need to do this. Perhaps another hole right through in line with the existing but into the other section? Could it be that there should be holes drilled into the bottom to drain this, second, partition?

 

No, no drain holes at the top. Well, not on the inside at least. Thanks for the help.