When I lived in Fair Oak in Hampshire one of the old houses had a Fire Mark over the door in the shape of a sun. In the 18th century this was to show that the owners of the house had insured with the Sun Fire Office. I think the idea was that if it caught fire the company’s private fire brigade would arrive and put the fire out. I assume that if there wasn’t a Fire Mark for that particular company on the building they watched it burn!!
Anyway there is an index to some of these insurance policies online. You need to go to the advanced Search on A2A at The National Archives website
and select the Guildhall Library from the repository list and then enter a “name” and/or “address” and the word “insured” into the all these words field.
I put in Pottinger insured and got 6 hits. One of which was this
Insured: William Pottinger, 27 Queen Street, Cheapside, tailor and draper. Other property or occupiers: Parish of Great Pornder, Essex (Law and others); Parish of Bloxham, Oxon (baker)
So enough to give you some idea if this applies to one of your ancestors.
There is a good leaflet about these records at http://18.104.22.168/NR/rdonlyres/E62730BB-5BFF-4ED7-8B7D-C7F4777A7805/0/48FIREINSURANCERECORDS.pdf
and an excellent article in the October 2011 edition of Your Family History magazine.