Sorry, yes you are correct.
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To amplify; although the signals reduced by two steps it was one step at a time, so the last green went G > YY and, after a pause as the train in front set back, YY > Y. as it was approached (if there was long visibility),- and all the other signals visible in the distance also stepped down. which understandably alarmed the driver of the following train. Stretches of the line South of Hanslope allow early sighting of signal aspects.
As an aside, the original 4 aspect signals on the line when cleared from R to G - after, for instance, a junction being cleared by another train in the opposite direction - went briefly but visibly R > Y > YY > G, I don't think this happens now. Presumably mechanical control involved