A windscreen chip won't be covered by warranty, so unless it would be an MOT failure (unlikely) it will be down to your negotiating skills.

Sale of Goods Act unlikely to be any help as the car is not unfit for purpose. Is it of satisfactory quality given its age and mileage? May be some leverage here.

I would have thought that they would repair the screen to a better quality, but a replacement may be a stretch. Worth trying for though.

Lesson: Don't inspect a car in the rain.

''I contacted the dealer today and he said he was aware of the chip but obviously didnt feel it necessary to point this out.''

That tells me all i need to know about the dealer, the only word missing was 'tough'.

If it annoys you enough you'll just have to get the screen changed, from the quote above i would expect nothing from this dealer now he's had your money, and i'd be giving them no further business eg servicing.

If you had raised it during negotiations maybe they would have done something about it for you. Probably would not have been a new screen, maybe just a better repair.

If the repair would pass an MOT there is no point in trying the Sale of Goods Act etc. If the car works and will pass an MOT its fit for purpose. Simple as that.