What is Church of England's doctrine on unbaptised dead infants and how was it formed? The penultimate rubric in the Book of Common Prayer Public Baptism Service reads: "It is certain by God's Word, that children which are baptized, dying before they commit actual sin, are undoubtedly saved." Clearly this statement refers only to baptised children.

My understanding (which could be wrong) is that throughout Western Christianity the traditional belief has been that, although we may have some hope in the case of unbaptised infants we do not have certainty. 

However in the Book of Common Worship, note 2 to the Emergency Baptism Service reads "[parents] should be assured that questions of ultimate salvation or of the provision of a Christian funeral for an infant who dies
do not depend upon whether or not the child has been baptized." This seems to say baptism is not needed.

Taking the two together it appears that the Church of England now teaches universal infant salvation regardless of baptism.

My question is, am I right in thinking this is now Church of England official doctrine, and if so is it a change and, again if so, how, by whom, and with what authority was this doctrine changed?
The Church of England is a wide-ranging denomination, and different parts of it hold different view on the nature of baptism. Many members prefer to delay baptism of children until they are old enough to decide for themselves. Others will see baptism as purely symbolic. I would be confident in asserting that there is no universally held opinion about the salvation of infants.

In your question you make a false assumption about the logic of the Church's statement on unbaptized infants. Stating that 'salvation [does] not depend upon whether or not the child has been baptized.' is not to state that all infants are saved. Salvation may depend on other facts, such as the faith of the parents, or on their intention to baptize (even if not carried out). 

Significant numbers of Anglicans might well believe that God would not condemn an infant killed before they had a chance to understand and receive the message of salvation.