I had the same problem with my 2004 Micra ignition and it was certainly worse in the hot weather. Wiggling the locked steering wheel was effective for a while, but one sweltering day nothing would budge it.
I think the moving parts had gummed up. A prolonged spray of carburettor cleaner straight into the keyhole got it moving and now it remains as good as new (three months on). I tried WD40 first, but that seemed to gum it up even more! I also charged up the battery for good measure, but it didn't need much charging.
If you try the carburettor cleaner, be warned that it makes the car smell of solvent for a couple of weeks and you need to unscrew the plastic wrapping around the steering column to tip out a puddle of any excess drained from the ignition. For me, it was a desperate measure before having the car towed!!
Patrick, October 2013