Found it!
When I was a spotty yoof, I was given for Christmas the 1966 Aircraft Annual (Ian Allen)
In it there is an article called "The First Generation" by Alfred Price A.R.Ae.S which details the development and use of three weapons; the X1, the Hs293 glider bomb and the X4 which was a wire guided air to air missile. The deployment of the X1 seems very risky, because of the requirement of the bomber to slow down on release to around 120 mph in order for it to arrive directly over the target at the time of impact. The bomb aimer would just need to keep the weapons` tracking flare superimposed on the target.
As soon as the Salerno bridgehead had been established complete with forward airstrips, spitfires arrived on the scene and the naval forces received the necessary air support, and German bombers could not maintain the steady and slow flight path required to make successful attacks.
"From September 9th to 19th 1943 the missile sank some 68000 tons of shipping and damaged a futher 90000 tons. For the remainder of the war, however, the X1 did not achieve a single success"