During 1940, Britain was frantically preparing for a German invasion which resulted in sections of coastline becoming heavily militarised. This field is located just 10km inland and therefore the grenades likely belonged to coastal defence soldiers.
The Home Guard were notorious for ill-disciplined storage and disposal of ammunition. Indeed, such finds are not uncommon in England.
The British Army designated the SIP grenade, the No.76 Special Grenade. It was a simple anti-tank weapon issued to Home Guard battalions throughout the UK. Britain produced more than six million SIP grenades.
Each grenade contained a mixture of white Phosphorus, benzine, water, and a strip of rubber. A glass bottle with a crown stopper provided the container.
On impact, the bottle would smash, the mixture would oxidise and an aggressive incendiary reaction would occur. This caused a substantial burn hazard and terrible injuries, which could be fatal.
The only way to ensure the greatest chance of identifying historic military activity is by commissioning a Brimstone Stage 1 Preliminary UXO Risk Assessment. However, UXO finds such as this are often not recorded in historical literature. So care must be taken when on site. Especially as these items do not contain any ferrous metal and are therefore difficult to detect.