Try spontaneous combustion - A substance with a relatively low ignition temperature can release heat, due to oxidation or fermentation. The heat cannot escape, so the temperature of the material rises above its ignition point then combustion begins if a sufficiently strong oxidizer, such as oxygen, is present.

Thank you all for your help.

Just in case anyone else needs to know this information I have been sent another reply that may be of interest. See below:

"Oxidative self-heating can occur in material contaminated by a variety of organic fluids, not just 'oils'. For example, a fire in 2004 in a sugar beet factory has been attributed to a plasticiser used in plastics manufacture."

Reference: http://www.hse.gov.uk/of...infosheets/is11-2008.pdf

The HSE publication 'The use and handling of flammable liquids' looks at how heating can be a source of ignition for flammable substances which may be of interest: http://www.hse.gov.uk/pubns/priced/hsg140.pdf

The following organisation may be able to advise you further:

Fire Protection Association: Telephone - 01608 812 500 or email - fpa@thefpa.co.uk

There are two possible reasons. The first is due to spontaneous combustion, this is caused by either biological or chemical activities in which the heat build up occurs faster than the heat is dissipated. Then a point will be reached where the temperature is sufficient to ignite the material so long as sufficient oxidiser is available. The second is due to the autoignition temperature of the lagging or contaminant being reached as a result of an external heating source. The external heating source could be the thing that is being lagged. In most circumstances this would have to be very hot, probably in excess of 250C.