The RAF acquired about1,000 of these the last being delivered in February 1942. Surprised production wasn't switched to something else once its weaknesses were exposed.
Looking at its performance as a night fighter for the period July 1940 to May 1941 some 100 Luftwaffe bombers were shot down by aircraft. The bulk fell to the Beaufighter - 46 with the Defiant accounting for 29.
One book says that the gunner was the commander of the aircraft which sounds unlikely to me.