The Defender Parts Catalogue doesn't list a screw to hold the lock barrel in place, it's a small pin that you have to depress to remove the barrel.
If yours had a screw fitted I can only presume that it was fitted by a previous owner.
Do you mean the little screw that goes through the plastic operating arm, through the housing and into the plastic key barrel extension?
If so then no I don't know what suze it is, I would just find a screw in my random screw collection.
As you can see I have found a screw but it may be a bit short. Previously I couldn't fully tighten the screw or the door didn't lock/unlock!
It seems I have a mismatch of parts!
That's different to the one I did last week on the Ninety. The key barrel end section was plastic along with the operating arm and plastic washers.
The screw was a self tapping screw since it was going into plastic.
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