This is caused by stupid design. The seat belt will not pull out unless the seat back is fully locked in the upright position ( because the belt is part of the seat back and, obviously, if the seat back is not secured neither is the belt and passenger ) but you cant get it into the upright position because the seat belt will not pull out!!!
Can you get at the bolt which holds the bottom end of the seat belt, if you remove that you can get the back up, release the belt then re-fit the bolt.
You may have to remove the seat back.
If you cant do it yourself take it to an independent workshop or body repair/panelbeater, they will be much cheaper than Vauxhall.
If the belt mechanism has been damaged by trying to force it, it might be easier/cheaper to get a complete second hand back from a scrapyard/breaker/ebay etc., but obviously make sure the belt on the replacement is in good condition.
In future plug the central belt in to one of the side connectors before folding the seat down so you have enough slack to get it back up.
laguna
January 2010