Ignition lock problem

I have a Rover 45 saloon on an 03 plate. The ignition lock sometimes won't allow me to insert the key. A close look reveals what looks like a tumbler or small steel barrel sideways on across the key hole gap. Fiddling around with a pin or hair grip usually puts things right for a while then it happens again. Has any-one any suggestions for a more permanent fix? By the way the 45 is excellent value otherwise and in my opinion really underrated.

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