OK - so digging further into this issue today, the problem appears to be that the Score Manager cannot return the Score for the Game, and instead can only function on a per-round basis.
So: I can use GetCurrentScore and loop it as Graeme noted, and that will return me for the Scorechanges in the current round.
When the next round starts, that value then gets wiped back to zero (along with the Hud widget from IslandSettings) whilst the M-key Scoreboard continues to hold the score for the Game (which is the Score that I want the Score Manager to return to me so I can align with the Hud widget).
There needs to be a GetRoundCurrentScore and GetGameCurrentScore. The Game Score is also the one that the Save device is holding onto making it even more frustrating.