Yale lock and subsiding wall

[ndrr]

hi,

I have a Yale night latch lock on my back garden gate. The gates hinges are on an exterior wall, the lock is attached to the side of the garage. The trouble is the exterior wall has poor foundations, and 'floats'. In the summer when the ground is dry it sinks, and now in the winter in rises up. The movement is enough such that the 'bolt' will not line up with the housing, and so the lock will not close.
I'm trying to find a larger housing to compensate, but no joy so far. Any help, or better ideas, greatly appreciated.

thanks
Neale