A cracked chimney is dangerous and a health hazard.
Contact your insurance and they will arrange a visit - carry our an assessment etc .
Get a quotation from a building contractor and send it to your insurance co.
Don't be silly - a burning pizza box did'nt crack your chimney.[/quote]
The most common reason for cracks to chimneys is chimney fire. These occur when the products of combustion condense on the inside of the flue liners ( ie, soot). An ignition scource is applied...ie, a burning pizza box whereby flames from the burning box goes up the chimney and ignites the carbon deposits and then we have a chimney fire. The heat causes the flue liners to expand and crack. However, if there is no room for expansion of the flue liners due to a compact infill surrounding the liners, then the heat from the blaze will cause expansion and cracking of the chimney breast and/or the chimney stack. So, the OP may not be silly afterall!!!
The OP should ask a chimney specialist to look at the chimney and advise as to the cause. In addition, if the OP had the house surveyed before purchase, any competent surveyor would have noted and recorded a cracked chimney. I would suspect that if the OP can provide insurers with the pre purchase survey which doesnt record any cracking to the chimney together with a report from a competent chimney specialist, then certainly, Insurers may consider that this is a valid claim for fire damage. I would stress that further investigative works would be undertaken by insurers before they would be satisfied that a policy liability arises.