PN
By 1943 the RAF new that the front and even mid upper turrets were seldom used at night. It dropped the latter for the Dams raid and later on both were often dispensed with. Nothing was ever done to counter the Schrage Musik Night Fighter attack profile for reasons that remain a mystery.
Another point worth mentioning is that some heavy nighfighters like the Me110 weighed down with radar and cannon had a heck of a time catching Lancs. Greater speed may have been more of a defence than four 0.303 Brownings.
The boffins did fit active radar to tail turrets but that only served to illuminate our aircraft as targets to the enemy once they had discovered this.