A chap I knew had an engine fire caption on short finals to a field, on NVG, and the student in the RHS confirmed there was a bright glow above and behind his head where the Stbd engine was burning. It took less than a minute to get the aircraft on the ground and to start shutting down, but in that time the port fire warning illuminated also! The aircraft was underslung out of the field and back to base where it transpired that the fire in the of the stbd engine bay had got to the point where the aluminium structure of the transmission deck was starting to liquify and the next event was probably going to be the TR driveshaft melting.
This fire was the result of a fuel leak from a high pressure pipe - hence the advice from the CAA to shutdown the engine if you are on fire - remove the source of ignition and you might make it to the ground, keep the engine going because you don't want to do an EOL and you might not have to bother.
My mate was lucky since he was close to the ground, he would have had to shut down one engine to use the extinguisher but elected to make an expeditious approach and landing on two engines. With hindsight he might have chosen a different option.