After reading the Department of Trade report into the loss of G-AGGF at Glen Lee 17 August 1943, the report states that no procedure existed at the time if an aircraft had to return to base and abort the mission.
One of the recommendations following the crash was to transmit a specific morse letter to indicate the reason for the abort, ie technical, weather or whatever, which seems to have been subsequently adopted.
When G-AGGF returned from it's aborted flight, a QTE was requested.
The Observer corps was unable to plot and send this information to the aircraft in a timely fashion, to prevent the crew from descending into high terrain to the north-west of Leuchars..
Presumably this was done by VHF transmission?
In addition, fighter units were stationed nearby at Arbroath and Turnhouse, so one assumes that IFF may have been installed to indicate the Mossie was a friendly.
Hope this information is of some help?