Complex sentence: A turret-less Valentine that had previously been used in a quarry was used by Greek militia it was fitted with an improvised armoured casement from which a gunner could fire a Bren gun.
Split into simpler sentences: A turret-less Valentine, that had previously been used in a quarry, was used by Greek militia. The tank was fitted with an improvised armoured casement from which a gunner could fire a Bren gun.