Write off, wow. I would definitely get onto Seat pronto. Does your insurer know you are planning on going to Seat? Have they/the assessor made any comment about it? I would definitely be getting a report/opinion from the assessing garage and/or the insurance company. If your insurance company are being funny about it, remind them that if you are successful and it is a manufacturer fault, then Seat are liable for the cost, rather then themselves.
I'd also bear in mind that Seat will want to see it for themselves, so if they insurance company want to write it off, bear in mind they will want to get rid of it asap. I'd think about whether you are prepared to buy it back off the insurance company if they don't want to play ball.
On the plus side, if the insurance co write it off and you buy it back, you might then be able to get them to repair it, and be quids in.