The excess is the sum of money you need to pay in the event of a claim on your policy. This amount varies from company to company.
Making a claim will not automatically lose you your no claims bonus. Most insurers will relegate you only two years meaning if you had 5 years no claims then you would still have 3 years.
Even if you have had a poor quote from a van insurer previously, try them again as sometimes their pricing structures change.
