Thanks to WW2 Talk, I now know what the remains of the mortar rounds I found at Westleton Walks are - 4.2" Mortars. These were introduced by the British in 1942, initially to fire chemical weapons but later both High Explosive and Smoke Rounds were developed. The British 4.2" Mortar was distinctly different from the American 4.2" mortar. The rounds I found were chemical rounds (presumably white phosphorus), part of Exercise Kruschen training???????
Image is a chemical round which I found on Visualcollector.com