Are we covered for a firework event if the fireworks are set off by committee members or volunteers ?

The policy will cover the committee for any losses which they are held responsible for, should an incident occur when a committee member or volunteer is setting off a firework.
The committee should ensure they conduct a full written risk assessment and comply with guidance from Health and Safety Executive and industry guidance.

Health and Safety Executive Guidance can be found at http://www.hse.gov.uk/explosives/fireworks/using.htm