That would be the 'P' Bomb. Certainly in use by the Battle of the Somme as it's mentioned in Operation Orders for 1st July. Basically white phos I believe. Cylindrical grenade with a pull igniter used for dugout clearing.
Cheers all the contemporary write ups I have read give a description of an item similar to a 2 pound firework containing wax and oil that was match lit. Pains Wessex bought them out but the records could be with Chemring in Derby who haven't responded yet to enquiries!