A member of the public stumbled across the two suspicious items (WW1 mortar bombs) and immediately called the Police.

On inspection, an MOD bomb disposal team identified the devices as British made Stokes Mortars. 

Police are urging people who use the common to remain vigilant and if they see a suspicious device, to withdraw to a safe distance, and contact the Police immediately.

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WW1 mortar
A diagram showing the Stokes Mortar weapons system and its ammunition
How dangerous are WW1 mortar bomb finds?

Expended yet unexploded mortar bombs are dangerous because they have been through the firing process and therefore a delicate detonating mechanism has likely failed. Any small shock could therefore cause the mechanism to complete its function and explode the main charge.

Mortar bombs are dropped into the launcher pipe base first. The impact on the base detonates the propellant charge, expelling the device from the launcher. It also ignites the delayed action fuse which eventually explodes the main charge once the weapon reaches its intended target area. Therefore, if an unused mortar were to be dropped in such a way, this could cause the bomb to explode.

A WW1 mortar found today is likely to be over 100 years old. Many decades of exposure to the elements will have degraded the item, leaving it fragile and vulnerable to impact. Furthermore, submerging high explosive in water does not render it harmless.