I have occasionally flown "flight tests" after major maintenance... such as "C" or "D" checks (I fly 747-200s) - problem is - I never asked if required, or necessary to do such... They tell me "you are scheduled for a flight check tomorrow at 1600, of aircraft OPA, parked on maintance ramp... - I do it..." - Then the next day they give me a flight check test items list, such as shutdown and flight starts, or alternate gear extension, etc... Fun flights anyway, sometimes takes 2 hrs in the air... for ridiculous little things (toilet flush cycles at FL 150...)
xxx
For certain (I think), a flight test is required when major maintenance is performed to control surfaces. As to for engine changes, I am not too sure if required or not. I remember a 3 engine ferry I did long ago (engine failure with severe FOD, big birds in nš 4 motor, fire warning and fire bottle discharged), we flew the plane back to Europe, and got another engine installed. The next day, we flew a live trip with cargo payload, no flight test... All I remember, is that we double-checked the nš 4 before takeoff...
xxx
For you in the UK, and military, might be different standards too...
xxx
Happy contrails