In that case it blatantly falls on the dealers head. Keep calm and know what you want.

1. Make it clear that you are expecting them to rectify the defect at their expense.
2. Point out that there is nothing on any of your paperwork to support the claim of sold as seen.
3. Point out that "sold as seen" carries no legal weight and the Sale of Goods Act 1982 states that goods muct be fit for the purpose for which they are sold and I beleive it also mentions that they should be of servicable quality.
4. If they don't agree to rectify the fault you will file a complaint with trading standards, have the repair done and take them to the small claims court to recover the sum and the associated costs of the temprorary vehicle whilst yours is being repaired.