Kitchen tap twisting?

Our tap has a movable arm but the whole unit has now become loose and the whole tap unit is twisting when moving the arm. I've looked under the sink and I see a spindle thread and nut directly under the tap. I've tried tightening it with fingers but its seems tight. There is no way to fit a spanner, it is directly in the very corner of the unit underneath and has the 2 water pipes in front of it with the gap being I'd say 2cms. I've heard of a flexible spanner but I'm not even sure it would fit in the gap plus the spindle hangs about 4 inches down and the nut is at the top. Any ideas and help welcome thank you!