[Arran]

Lightning could have been the culprit, if it looks burned, or some form of mechanical damage to the winding. The tubes themselves seldom short, and the shorts are usually between one grid, and another, or between the heater and cathode. What can cause an IF winding to fail might be the IF amplifier tube being biased improperly drawing excessive current through the primary, possibly due to a leaky capacitor, which is usually connected between the B+ and the plate of the IF amplifier tube. Sometimes they fail for mechanical reasons, like the wire physically breaks, or corrodes.
Regards
Arran