In reply to fast eddie:
If you have something worth stealing they will probably be back in a few months to get whatever you have replaced it with. However, I think it depends if it was an opportunistic theft, or a more calculated one.
Thieves will go to great lengths to nick stuff. Our neighbour had his quad bike stolen a couple of years ago. It was locked in a steel framed building out of sight of the road. The thieves knew exactly which building to break into. I can also think of several other local thefts of plant and equipment where the thieves knew exactly which building to break into. This is in the middle of the countryside from buildings which aren't visible from the road.
Even worse if you catch them red handed and can actually identify the thieves, it seldom seems to result in a prosecution.
Like others have suggested CCTV and maybe fitting a tracker to a high value bike is probably your best bet.