Just to amplify the Gutersloh Harrier strength. From 1977 to 1980 these were the aircraft on 3(F) - total 21, including the two T4s
A XV741. K XW763 W XV808
B XV738. L XW917 Y XV801
C XW764 M XV751 Z XZ139
D XW765 N XV792
E XW766 O XV781
F XZ132. P XV779
G XW630. Q XW933
H XZ138. R XV793
J XZ134 T XZ145
Of these “Y” crashed Dec 78, “O” Jun 79, “E” Oct 79 and “D” Mar 80. So for the whole of 1977 and 1978 there were more than 18 and less than 24! I would assume 4 Sqn numbers were the same.