+1 for Ed, once you've got a feel for the joint concerned and it won't stop leaking strip it down and rebuild with a hydraulic sealant, Loctite 542 or similar.
Remember spanners are made to a certain length, proportional to their size, this is to help prevent over enthusiastic tightening!
Combination spanners allow the wise engineer (and evil bastards, such as my self) the luxury of clipping two together to double the force in times of dire emergency............
Cheers
Stef