you don't buy them yourself, go via your insurance. Be just be the excess on your policy, normally anything from £30 to £100
do they always go on the cornner, is there 2 layers to the glass as top of the glass has no crack looks like its the black part? on full cover but says to shop around and they will do the rest but trying to find a ruff price they gave me a list auto glass etc they just want a quote for the price.
All modern cars have laminated windscreens, which is basically 2 thin sheets of glass, with a sheet of flexible material in the middle. This stops the glass shattering into a million bits like the old toughened screens used to.
Most cracks in screens happen as a result of an impact from a stone or similar. However Omegas do suffer from stress cracks in the bottoms corners of the screens. This can be because of 2 reasons...
Usually it's delamination, caused by a combination of moisture ingress between the layers of the screen, and age causing the screen to become brittle. Eventually it will cause the glass to crack, and is often joined by a milky appearance to the glass in the same area.
The other cause can be rust developing in the area where the screen is bonded to the car. As the rust takes hold, it expands, pushing against the screen until it cracks under the pressure.
If you find the latter, it's imperative that you treat the rust before the new screen is fitted. Otherwise the new screen will simply crack again, and you will almost certainly get water ingress into the car.
Hope this helps.