The Collision Damage Waiver usually excludes glass and tyre damage. Some hire companies will offer additional excess waiver (when you take out the hire agreement and collect the van) that covers these shortfalls, but this usually costs extra. As a rule, we always recommend taking out independent excess cover as this will usually be much cheaper than the product being offered on-the-day at the rental location: vans.honestjohn.co.uk/cheap-van-hire/

That said, I'd be annoyed if I was asked to stump up the cost of a new windscreen because of a stone chip. Sometimes national hire firms will waver the cost or reduce it if you make a complaint. If the van hire staff never mentioned the additional cover when you took out the agreement then use this as your basis for a complaint.