Smoke alarms - should be fitted on each level and especially in the roof space.  Should be tested regularly  There are stand alone or linked options.  Professional advice may be sought.

Fire alarms and chimney heat alarms are also useful early warning systems.

Or keep an eye on the flue temperature with a flue thermometer.

TAC Fire Safety Advice pdf Sept 2010

Burgoynes Fire in Thatch Investigation Results 2011

Burgoynes Forensic Investigation of Thatch Fires - results showing that of 61 investigations, the most common cause they identified was ejected embers (40/61).  This challenges the heat transfer through chimney structure to the thatch report from 1998.