Hooray, the bench seat issue is almost resolved. Had an update call from Stuart at Citygate this morning. Removed the seat and discovered the issue was twofold :-
1. The connecting linkage between front and rear release catches had become detached (as I suspected)
2. The rear catch was extremely stiff (which may have been the cause of 1.)
Anyway complete new seat frame going in this afternoon - apparently VW doesn’t supply individual components for the seat frame! (Mad)
My van should be ready for collection from this evening
Big thumbs up from me for Citygate Wooburn Green, they have been totally professional in dealing with this very out of the ordinary issue.
I can now obsess about something else