The fitting may have become loose or not centered properly. Obviously the best way would be to be able to access it from under the bath and refit it properly. However you could try the following although the fitting may not be as I describe.
In the chrome cap you should find a small hole with a grub screw inside, this screw holds the cap on the spindle. Before loosening this screw make sure you cover the plughole with something in case the screw comes out and you drop it. Loosen this screw until you can pull the cap off the spindle. Underneath in the center will be the spindle and also a nut of some kind that holds the fitting to the bath, normally the nut has a hole where the spindle comes through, around will be the overflow plate with holes in it. There may or may not be a rubber seal between this plate and the bath but outside of the hole will be a rubber seal and then the outer assembly that the nut screws in to. If you look through the overflow holes you may be able to see the rim of the hole in the bath that the overflow assembly fits through. If the overflow assembly looks fairly central then tightening this nut may seal the assembly better. If the assembly is off centered then loosening this nut may allow you to tease the assembly up/down/left/right until it is centered. However if you loosen it too much then you could detatch the assembly completely and then you would have to take the tiles off to reassemble, so do so with extreme caution. I would try and tighten it first to see if that seals it.
In the chrome cap you should find a small hole with a grub screw inside, this screw holds the cap on the spindle. Before loosening this screw make sure you cover the plughole with something in case the screw comes out and you drop it. Loosen this screw until you can pull the cap off the spindle. Underneath in the center will be the spindle and also a nut of some kind that holds the fitting to the bath, normally the nut has a hole where the spindle comes through, around will be the overflow plate with holes in it. There may or may not be a rubber seal between this plate and the bath but outside of the hole will be a rubber seal and then the outer assembly that the nut screws in to. If you look through the overflow holes you may be able to see the rim of the hole in the bath that the overflow assembly fits through. If the overflow assembly looks fairly central then tightening this nut may seal the assembly better. If the assembly is off centered then loosening this nut may allow you to tease the assembly up/down/left/right until it is centered. However if you loosen it too much then you could detatch the assembly completely and then you would have to take the tiles off to reassemble, so do so with extreme caution. I would try and tighten it first to see if that seals it.