The answers have been really helpful and informative and of course the these answers always seem to through up new questions as a consequence.... thank you for you patience on this.
Another question regarding cores.
I have an old TV flyback minus the turns and air gap so just the core. Using two of these, in an "E" formation (next to each other), could you conceivably use it as an SMPS core... I know they operate as a flyback at, I think, a frequency of ~25kHz so the frequency seems correct for an SMPS... are there any show stoppers with this?
One side is square and the other leg is round... if you put two together I suspect you would use the square leg side with another core to ("E" format) and wind on the turns on this square leg)... will having the other leg being round have a detrimental impact on the system?