Ford Fiesta V reg Gear Change

My gear lever, suddenly, no longer returns to the "centre" position. I can change gear although care must be taken to select third rather than fifth.
My local ford dealer has suggested that I need the entire gear change assembly replaced whilst an independant garage suggests the fault is not in the linkage but in the gearbox itself. Has anyone come across this before please? More importantly what do I need to fix it?

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