The photograph shows the quantity of "pot-boilers" I found when excavating the Iron Age ditch feature in previous years. At the time it didn't seem unusual but when you carry on digging elsewhere and find none of them you get a better perspective. On Time Team they get excited when they find one!
Apart from one or two examples of whole, smooth, small stones, all the rest show signs of being pieces that have cracked away from larger, smooth stones due to thermal shock.
The quantity seems to indicate use and disposal over time. The ditch they were found in showed signs of having gradually filled up.