This happened to me once when i was in Manchester. No matter how hard i tried the key would just not turn so i called the RAC. They did the same thing i was doing with no luck so he got some metal rods out and made me sign a piece of paper allowing him to hammer my steering column from underneath.
After a few hard nocks he managed to free whatever was stuck, hey presto the key turned. Said that key chamber needed re-greasing because it has dried up.
I just left it and had no probs for about 3 years now.
Hope this info helps.
Angel, September 2011