Well, the rigger in question has got in touch with me directly, thank you to him and all others for all your help.
He left the kit with the drop zone at Sibson, and like me, probably would have expected that to be safe. (It always seemed very reputable).
I can't really find much fault with the rigger for this - at least, I'm sure other riggers would do the same at a supposedly trustworthy drop zone.
However, the rig, which at the time was all in date with AAD probably doesn't just vanish from a drop zone without someone selling it, theft, or it being lost in an attic for years. The latter would be nice, but seems unlikely.
It would be helpful if someone with knowledge of Sibson from the last four or five years could explain what has happened there during this period so I can get in touch with both the previous and current companies there, especially now I have a reasonable written explanation from the rigger.