How to tighten nut under kitchen sink

My kitchen tap has come loose from the sink and I'm having trouble tightening it back up. This is what it looks like.

I need to tighten the nut, but the problem I'm having is there is not enough space between the nut and the water hoses or the sink basin (Bottom of the pic) to turn the wrench when holding it parallel to the nut. I tried turning by putting the wrench perpendicular to the nut, but the bolt comes down to far to allow me to do this. I've also tried using pliers the same way, there is enough clearance to with the pliers to turn perpendicular to the nut, but the nut is so thin I can't get enough grip to tighten it much, I managed to tighten a bit this way, but it came loose again after a few days.

I guess I need to buy some sort of socket wrench, but not sure what kind. Or is there some other tool I should use?

Edit:

I have tried tightening using the a screwdriver in the flat slot of the bolt, that seems to secure the bolt to the tap. The problem seems to be the tap and bolt are not secured to the sink properly, which I think is what the bolt is doing.