Rear seat - how to obtain parts - and ridiculous Seat Customer Service

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Can anyone help me find the replacement part pictured. It's from my Alhambra's rearmost seat and links the folding seat cushion latch to the seat back. There are two on each seat. One has broken off (and disappeared) and the result is that the seat cushion no longer stays in the seated position but springs up into the fold-away position. Both the Seat dealer and the Seat Customer Services claim that the only way to fix the problem is to replace the entire seat (and quoted over £500) as the part is not available separately. They also claim that even if it was, the seat would have to be removed to fit it. I have just removed one from the other (fully working) seat.. it took me approximately 3 seconds, but then what do I know..? I was also told that my Seat extended warranty doesn't cover seats as seats have springs in them and springs are excluded... but that's one for the insurance ombudsman

Any pointers gratefully received.