If it's an accumulation over the years, the insurance company isn't responsible - it comes under wear and tear. That's fair enough - you wouldn't expect them to pay for new tyres when they wear out. However, if there is one crack or chip that you can identify its origins AND it would make the car fail the MOT then they will (or should) pay up.
How you do it depends on the company, but most windscreen outfits like Autoglass will come out and do the job, just collecting the excess from you (usually �60 but check your policy/certificate) and billing the rest direct to the insurers.