In reply to rallymania:
The sad thing is that no matter how much you lock bikes up, if someone has the time and determination they will probably get the bikes.
I think the best deterrent is multiple levels of security that provide hurdles delaying the thief in vulnerable open spaces.
If a thief can sit in the garden for hours without being noticed slowly picking their way through the security devices they will get the bikes.
I have a friend who keeps 3 bikes with a combined value of ~£4,500 in a shed at the very end of her garden with only the most pathetic little hasp and staple on the door. I told her that she should be a bit more careful but her response was that it was a nice safe area; you can't win with that logic especially as thieves go to the nice areas to steal stuff but they know that there will be good stuff the steal.