Thanks for the feedback guys.
Another issue discovered (though its working pretty damn well and I'm loving this!).
I'm using displacement map for the waves in this pool. It's working well, very fast. However, I'm guessing the calculations of the displacement are causing problems with the projection of the caustics. See two attachments - the first (camera) is incorrect, the caustics stop (near the black line - coincidental), however, if I change the view slightly (so more of the pool is visible) and restart the render, the caustics renders properly. If I then switch back to the camera view (without restarting the render) the caustics renders properly from the camera.
I hope this makes sense - happy to share scene if helpful. Any suggestions how I might overcome this problem?