How to fix a Mountfield Empress 16 fuel leak?

Hi,
Went to use my mower and the petrol tank was empty. It was not empty when I put it away last. So I put a small amount of fuel in the tank and discovered it was pouring out of a small scew at the bottom of the carb. The screw is difficult to get to but it feels like it has a posidrive type head. Although there is no room to get a screwdriver under it.

The scew has a spring around it which is under tension. I have tried to finger tighten it but it does not seem to catch on anything? Anyone know how I can fix this? I have found a diagram of the mower on the net but the engine is not shown in any detail.