[PaulR]

The transformer consists of two coils formed round some sort of plastic or (I suspect) cellulose tubes. The tubes sit on two pegs within the can - I am not sure how they couple like that but it must have done

The core in the offending coil had been pushed right down to the peg and the slot in the top of it was a bit chipped. The core was stuck like that. I tried some switch cleaner to soften any wax but when it freed the core just turned round without moving up. Eventually the former split apart and the coil is now o/c.