[tcolvin]

Nick,

We know that,
a) the XX Committee believed (in January 1943) that CARELESS was "thus able to instil into the Germans a wholesome fear of the strength of our passive defences" (Masterman), and
b) "the Luftwaffe High Command now (April 1943) persisted in the belief that daylight Jabo missions had not achieved the desired effect and therefore the vast majority of SKG 10 would continue to be trained for nocturnal attacks, to the incredulity of Jabo pilots" (Goss).

To answer your question about how this wholesome fear was assessed, we would need to see the detail in M.I. 5 records on which Masterman based his statement. That record would consist of Enigma intercepts since, according to Masterman, that was how the XX Committee checked the effect of its activities on the Germans.

Chris Goss is one obvious person to study M.I.5 records, and to find out whether XX was the source of the German High Command's developed sense of failure.
You are another obvious person.

You do not, I assume, have a third theory why the daylight Jabo missions were abandoned? Chris Goss believes it was probably due to "incomplete intelligence", which is not a million miles from my hypothesis that it was probably due to XX deception.

The RAF would have won if they had wiped out the Jabo force attacking London, because thereby they would have instilled into the German High Command a wholesome fear of the strength of the "active" defences.
Since, however, that proved to be beyond the capability of the RAF, then the only other way of getting the daylight Jabo attacks to stop, was XX.

Tony