The date sequence and content of these letters confused me until I realised that there must be additional correspondence which has not been published.
Annette Whittemore wrote to Prof McClure on 12 April and this letter was published.
McClure's reply is not dated but says "When your email of the 23rd April arrived...." and "I was appalled and saddened to read of your daughters medical history".
This implies that Annette Whittemore sent a subsequent email to McClure on 23 April, possibly a more personal one, appealing to the Prof as a mother perhaps (do we know if McClure has children?).
Annette's second letter to McClure, dated 27 April, says "I appreciate once again your timely reply", which implies that she has received more than one reply from McClure.
Perhaps McClure replied more formally to Annette's initial letter dated 12 April and Annette's email of 23 April was in response to this but neither have been published.
It will be interesting to see the outcome of the Freedom of Information request that has been submitted to Imperial College London regarding this correspondence.