Apologise, bad ettiequte from myself.
Whether it's a solution or a hack is to be seen.
I decided it would probably suffice to:
* turn the Map Sequence object back to the list of maps
* use the insert attribute of lists to place the new frame (converted to a Map object) in the desired location
* then covert back to a Map Sequence.
Maybe there is some unnecessary overhead in this. So if there is a cleaner way, I would also like to hear it.