Kangoo steering lock

My recently acquired Kangoo diesel has been working just fine - and suddenly the key that I normally use won't go into the ignition lock or when it does it is just stuck and won't undo the steering lock. My spare works fine.
Is this a low/poor battery issue or to do with the steering lock?

The key works about one in every fifteen tries and is driving me crazy.

Regards

Kangoo steering lock