I was led to believe (by BWOS) that, in the early 80s, the installation of the laser fairing on the GR1 made one of the engines (the right one? ) rather prone to surging at lowish Mach 1+ speeds (I don't know whether this figure was classified so I can't be more precise). Perhaps the FLIR fairing on the GR4 had the same effect on the other engine? Whatever, the TTTE Tornados, not being fitted with those fairings would not have had the same problem.