Sorry for not being informative enough in the original post
A Boris Bike is the nickname given to the relatively new bike hire system in London which is run by the Transport for London. You can find information on their website and sign up.
There seem to be lots of different tariffs for usage but I know that after an initial access fee (£1 for 24 hours), you can use multiple bikes up to half an hour for free on each bike.
I found them a good way of getting between cache areas in London - docking the bike at a nearby docking station, hitting a few caches then picking up another bike to move on. Using the tube system before, I didn't really get a grasp of where everything was in relation to eachother, but with the bikes I'm getting to know my way around the city a bit more.
I recommend doing the following BEFORE undocking a bike:
A check of how freely the back wheel spins is a good idea - some of them can be a bit sticky which will make your journey harder work!
Check the brakes work and also check for any other damage on the bike (pedals etc)
Adjust the seat to where you need it.
Check what time you're starting so you know how long you've got until the pay increase.
I've had a couple of bikes with lights that don't work. I'm not sure if it's possible to check these before departure as they run on a dynamo.
If you undock a damaged bike, you can dock it again but you'll have to wait 5 minutes before you can take another bike.
Also, make sure your bike is docked properly when you put it back! If it isn't, you'll continue to pay for the usage on it, or it might be pinched and you'll have to fork out a fortune!
Hope that helps