potato goose

Really sorry for you! I had mine written off at the start of January having just bought the car - so I feel your pain.
1. I second everything OllyW says. Make sure you get the witness to follow through on their statement so you make the fact you're not at fault as concrete as possible. If it's sensible drawing diagrams of what happened might help make the situation totally clear cut?
2. Valuation: this is hard, as the insurance companies will probably base their valuation of the car on trade guides (Parkers, Glass's, CAP), and it's a serious uphill battle to get them to deviate from this. They tend to take this page as their guidelines: https://www.financial-ombudsman.org.uk/publications/technical_notes/motor-valuation.html
Mine was an aero, so rare and the trade guides did not give a fair representation of price to replace it, but trying to tell them this was like headbutting a wall.
Also important, their valuation is the basis of whether it's written off...
3. You have the right to choose where it goes for repair - and choose carefully. You can probably influence whether the car is written off as you'd be amazed at how big the difference in repair cost can be between garages. As an example, the write-off value for mine was if repair was >66% of their valuation (this will vary between companies). BUT if you choose somewhere that's not one of their 'approved repair centres' (read: industrial chopshop) it will affect their liability for guaranteeing the quality of the repair. Also, if they are presented with multiple quotes, they will query any cost discrepancies.
4. Get photos of literally everything. Even bits of the car that you don't think are relevant. I was rear-ended, and during recovery a tow truck crunched the front bumper. When I told the insurance company that I expected them to handle this repair, they responded that unless I could prove photographically that the bumper wasn't damaged prior to recovery, then they wouldn't pay to repair it. Any damage that can't be explained by the specific circumstances of the accident, they will try not to pay out on - so I'd try to work out what your mechanical damages are, and figure out how the accident caused it.
5. Claim on your insurance policy. If the van driver's insurance contacts you to settle, don't accept. If you claim on the other driver's policy to protect your NCD or save on excesses, you lose the right to complain to the financial ombudsman about their handling of your claim, as you are not the person the insurance company has the agreement with. Then when they undervalue your car, or mess you about in any way, you cannot have the claim arbitrated by an independent body. My understanding is that in a non-fault claim if you claim on your own policy the insurance company should recover 100% of the costs (including excesses) from the other side, so you shouldn't be hit by those. Although I'm not 100% sure on that - something to ask your insurers about.
Sorry for the essay, but I hope it helps and hope you get it all sorted out as best possible!

Hi Everyone,
Finally converted from a long-time forum spectator to a real life owner! I recently bought a red Aero, and instantly became the butt of every fast and furious joke or 'Do you take that tea tray around so you can have a picnic anywhere?' comment that my family can think of. Don't really care though, I'm pretty delighted with the car.
So I'm looking forward to many happy miles of GT86 ownership, meeting other members up here in Scotland, some track events/time, and of course small village pariah-dom, ahead!

Hi Everyone,
Finally converted from a long-time forum spectator to a real life owner! I recently bought a red Aero, and instantly became the butt of every fast and furious joke or 'Do you take that tea tray around so you can have a picnic anywhere?' comment that my family can think of. Don't really care though, I'm pretty delighted with the car.
So I'm looking forward to many happy miles of GT86 ownership, meeting other members up here in Scotland, some track events/time, and of course small village pariah-dom, ahead!